Here's the deal: Garen's not the tank everyone plays him as - you can be so much more. This Garen build will guide you to being not only an immovable force in the middle of their ranks, sewing chaos wherever he goes, but also an unstoppable killing machine. I've looked at many guides and read many builds, but I've built this one from the ground up, and without wishing to boast, I regularly achieve an 80+ % win ratio as the Might of Demacia, and it's been an awfully long time since I've gotten less kills than deaths.

For my Quintessences, I went with Greater Quintessence of Desolation - I found that there was always a massive DPS increase upon upgrading to Last Whisper, purely because of the armour penetration. Even against casters, these three runes alone will allow for a 4% DPS increase - doesn't sound like much, but this amounts to an extra 50 damage or so per spin per enemy champion hit, and against more beefy tank classes, it only gets better, about a 7.5% increase, lending to a possible 90 or so per spin.
The Marks went to armour penetration too - the aggregate of the action of the nine Greater Mark of Desolation lends itself to an even greater DPS increase than the Quints - 6% or so on 'squishies' - about 65 per spin - and somewhere in the region of 10% on tanks - around 125 extra.
Since Garen is often the focus of enemy fire, and Glyphs and Seals are for the most part useless offensively, I used these runes defensively. I took Greater Seal of Evasion for my seals - this, combined with the mastery Evasion allows Garen a total of 8.75% dodge chance, which against physical damage dealers, specifically carries, adds up to an even larger percentage damage reduction, as dodging one attack can prevent the proc of Vayne's Silver Bolts, or Twisted Fate's Pick A Card, for example. These dodges will also proc the effect of the Nimbleness mastery, allowing for a rapid escape in tandem with Decisive Strike, or an easier hunt.
The Glyphs were then chosen as Greater Glyph of Magic Resist, as these combined with the mastery Resistance allow for strong magic resistance in the early game, before Garen has the opportunity to access health items such as Frozen Mallet, keeping him alive in the early game and permitting heavy harassments against characters who would otherwise deal crippling damage to him in any other circumstances, such as Annie and Brand. Late-game this magic resistance will also allow for better survivability against items and effects that deal maximum health-based damage such as Madred's Bloodrazor.

--- PREVIOUS RUNE SET-UP ---
Time gone by had it that I ran Fortitude for the Seals ( Greater Seal of Health) and Glyphs ( Greater Glyph of Health). Health is a very general statistic, and although it doesn't make a massive amount of difference in the late game, the extra 72 health from these 18 runes makes a huge difference in the early game - it's the difference between being able to chase and dive or not, and surviving that last spell, or Vayne's last shot between the shoulderblades bouncing off harmlessly during the crucial "First Blood" phase. The reason I chose these over the corresponding glyphs ( Greater Glyph of Vitality) and seals ( Greater Seal of Vitality) of Vitality was due to the fact that these supply much greater early-game survivability before you have the chance to access Phage, Frozen Mallet or Warmog's Armor - this is the stage at which Garen is at his weakest, but at competitive levels of play by no means is it the stage at which pressure is off - Garen is known in ranked and tournament level as a powerful late-game presence, and the enemy team will often choose to focus-fire him in the early game to prevent him from becoming too powerful - these runes help to inch you along until you're out of these troubled waters.

I finally decided against this set-up due to the uselessness of these runes late-game. In total, they add less health than a single auto-attack will deal at endgame, as opposed to the current set-up, which allows for moderate survivability versus magical damage and strong survivability against AD carries, as well as the offensive/defensive component provided by Nimbleness .

This is where most people will be questioning my judgement (oh, how punny) as to going what seems like a very odd combination of items. I'll take this opportunity to explain each of them in turn, along with other possible choices.
Doran's shield. Pretty standard, Garen's recommended starter. I figure that in the early game, you don't need Boots right away, as Decisive Strike gives you all the speed advantage you need on your opponents, and Judgment does massive amount of damage early game without needing Doran's Blade to back it up. The health and health regen this shield offers is ample for Garen to throw himself in there early on without fear of reprisal, and only a short regen time, coupled with his passive.
Boots of Speed. With Garen's regen keeping him near the fight indefinitely in the early game, it will be a long time before you have to, or want to, go back. When you finally do, you should have enough money for Boots, to keep up with opponents who will mostly have gone back and bought them by now, and Phage, as below.
Phage. This should be bought along with Boots on your first trip back - the extra health it lends is crucial to keeping you alive, along with those first ranks of Courage as you spin in there, and the slow gives you a way of hunting them down, as well as slowing them so that the rest of your team can catch up. The damage bonus allows you to just about keep up in DPS for a while until you access the later items.
Boots of Swiftness are a must - Garen's strength relies on his mobility, and being able to hit Decisive Strike quickly to get to the middle of the fight and Judgment is crucial. They also aid his quick escaping - Decisive Strike, Courage and Judgement allow Garen to slide his way out of near any situation, along with Ghost when so necessary.
Frozen Mallet is here as of the same reason as Phage - the 100% slow is immeasurably useful when hunting people down, and the health is what will keep you alive in metagame fights until you reach Warmog's Armor later on, allowing you to really throw yourself in there.
Last Whisper is here at this precise stage because this is the point in the game where the ante, as far as armor goes, starts to get upped. This should be the point each game where tanks start to buy their heavy armor items, eg Thornmail, and we see the Aegis of the Legion appear, which can drastically reduce your damage. Upon buying Last Whisper you should notice a significant increase in your DPS - one several times greater than will be applied via the runes mentioned earlier.
Infinity Edge comes next, significantly AFTER Last Whisper. I found this the optimal sequence, as the Infinity Edge is considerably weaker without the armor penetration than is desirable. Infinity Edge should exhibit a skyrocket in your damage, as both Decisive Strike and Judgment benefit from your attack damage, but more significantly Judgement benefits from Critical strike - you will notice a small red icon above their heads when Garen scores a critical strike - these will become commonplace once you have this item and begin to spam the spin.
Warmog's armor, as this is the stage where their carries begin to become extremely dangerous. You're the frontline man as Garen, and will often be the focus of much enemy fire - you need survivability if you're to charge right in there and spin to your heart's content, and the health offered by Warmog's is a pretty generic way to do it - it doesn't matter if they're AP or AD, health will keep you good through it.
Atma's Impaler finishes off the set - sell your Doran's Shield and buy this, as with around 4000+ health at this point, you will notice around an 80 Attack Damage increase, really bringing home the damage of Decisive Strike etc, and greatly increasing the damage of Judgment, due to it's reliance on critical strike for damage.

Now, for some greater survivability versus an AP heavy team:
After Infinity Edge go for -->
(Note that this build is considerably more tank-heavy, and should only be considered in the case of at least 4/5 enemies playing AP, and if they're focusing you. It's also worth noting that this build is particularly strong against champions like Udyr and Warwick who all too often sport the tank-killer Madred's Bloodrazor - Upon seeing one of these on the enemy team it's advisable to swap out at least one of your items (I would normally choose to forgo Atma's Impaler) for some form of magic resistance - against the aforementioned champions it's more useful to take Banshee's Veil as it will protect you from Infinite Duress and the powerful DoT effect from Wilding Claw's activation).

Against an AD heavy team:
After Infinity Edge go to -->
(As before,only for use if you've provoked the attention of the enemies, and you're up against a lot of AD carries, such as Jax, Vayne or Xin Zhao).
Notice in the above builds how I keep the build the same until after Infinity Edge - this is because the Frozen Mallet gives you the survivability to reach this point, and in order to keep up the damage late game, you really need the Infinity Edge.

If you're being fed heavily early game:
--> --> --> --> --> --> -->
Sell the Sword of the Occult to buy the Atma's Impaler end-game.
The Leviathan's bonus health will account for the lack of HP from the early Frozen Mallet and the lack of Warmog's Armor, though it may be worth replacing Leviathan with Warmog's late game, if you find yourself wanting for health. The extra damage from the Occult will tide you over until the Atma's.
It's worth noting that you're in a dangerous place as Garen in each fight, and if enemies see you rolling two snowball items they may choose to target you - as such it's not a build I would recommend unless you're feeling extremely comfortable with your position in the game, since one or two deaths will have dramatic effects on the effectiveness of Leviathan and Sword of the Occult - these items are great early-game but the faster you are able to move on from reliance on them the better.

On Phantom Dancer and it's omission
Unlike many other DPS guides for Garen, you will notice mine fails to contain what many see as a core component of his build - the Phantom Dancer. The reasons for this are thus:
-The movement speed aspect of this item is not required for Garen. It's well documented that as a DPS Garen you should certainly be rolling Boots of Swiftness - it's a staple item of the build. Together with the speed modifier of Decisive Strike, you will find yourself able to catch anyone who you would catch, or flee from, if you had Phantom Dancer, but no better off against those you would not. For example, Garen with Boots of Swiftness and Decisive Strike will be able to chase down characters such as Amumu, Karthus, Fiddlesticks et cetera, but he will not be able to catch those with escaping mechanisms, such as LeBlanc, Ezreal and Corki. This fact will NOT change with the inclusion of Phantom Dancer, and so the movement speed bonus from it goes mainly to waste.
-The attack speed inherent from Phantom Dancer does greatly increase his damage output when Garen's abilities are all on cooldown, but this is not, and as I view Garen, should not be an issue. Garen is primarily a burst DPS character - he can deal ridiculous amounts of damage with both his Decisive Strike and his Judgment - easily two thousand within the first four seconds of a team fight. The cooldowns on both of these abilities are short at endgame, and thus there is little downtime, and little need to increase his damage in this period with attack speed. Furthermore, the attack speed is wasted for both of the aforementioned abilities, as it increases the damage of neither of them. Though the attack speed may be useful for tower killing or dragon/Baron hunting, neither of these should be a concern for Garen, especially as his Decisive Strike allows for sufficient damage to all of these targets.
-The critical strike is, in my opinion, the best aspect of this item. The 30% increase can be seen as an effective 45% increase in Judgment damage, when combined with the passive effect of Infinity Edge. However, as I will now explain, getting this item for the Critical Strike alone is not an effective use of an item slot.
-Finally, Garen simply cannot afford the inventory slot for this item. All six items are required for the effectiveness of this build - feasibly, the ones which could be swapped out are Atma's Impaler, Frozen Mallet, Warmog's Armor or Last Whisper. However, swapping out either Frozen Mallet or Warmog's Armor will drastically reduce the effectiveness of Atma's Impaler, a cornerstone of this build in the endgame - removing Atma's Impaler will cripple the damages of both Decisive Strike and Judgment, and excluding Frozen Mallet will impede Garen's chasing potential - with it, he is able to catch an opponent out of position with Decisive Strike, and then use the slowing effect of Frozen Mallet to allow friendlies to catch up - without this, you will find many enemies escaping the grasp of you and your allies.

Why I chose to max out Judgment first:
My thinking behind this was that this is easily Garen's primary method of damage, and this is of course a DPS focused guide to the Might of Demacia. Against faster heroes, it may be more sensible to forgo a point or two of Judgment to put some more in Decisive Strike, in order to chase them down or dive in and out of the bushes for harassment, not to mention for quick escapes.

Why I alternated between Decisive Strike and Courage:
The reason for this is that Garen is always in the thick of it, and as such is often the focus of frantic enemy fire; the ability to chase suddenly is much less necessary than the reduced cooldown and increased survivability offered by Courage. It's still necessary to keep up with Decisive Strike though, as it allows for quick initiation and escape, and for you to keep up with enemy champions while in Judgment, as Decisive can be popped while spinning.

With this build Garen fulfils several roles within team fights. Crucially, you are not the initiator - if you see an enemy out of position, it is not your job to run at him and get things going - it is much better to rely on someone else who has some method of crowd control to get things started. Most team fights should begin the same way - someone on either team is caught out of position, they get stunned, melee champions run right in to that champion, ranged stay out of reach and bombard it from afar. What you're definitely not supposed to do is just run into the brawl - Garen is wasted here. Ideally, you should hit Decisive Strike and sprint straight past it, bringing yourself straight into the middle of the enemy ranged champions. The Decisive Strike effect should still be on, so before anything else, upon reaching them you must use it to silence an enemy caster, or failing that, somebody who is reliant on their abilities or summoner spells (remember that silence will prevent the use of Flash to escape). If you have reached the point in the game where an enemy carry is becoming notoriously dangerous, it's often worth Exhausting them to reduce their damage output significantly. This shouldn't take any time at all, and so now you can hit Judgment, and start racking up the damage on the weaker enemies you've found yourself next to - the beauty of it all is that because of your silence and possible Exhaust, they shouldn't really be able to stop you until after Judgement's over, but in case their melee characters turn on you or you start to take notable damage, pop Courage to keep yourself in there. With any luck, you should be ahead of the game item-wise, and so you may even have done enough damage to one of these opponents to weaken them to Demacian Justice-able health - if this is the case, it's normally advisable to pop it right away, but note that sometimes it's better to save it for enemies with high max hp and low MR, such as Cho'Gath, as using it to finish him from about 25% health is a great way to free up your team to do other things rather than labouring through that last thousand or so HP. If you have done everything right, as you Judgment for the first time, it should scatter the enemy ranged until they can find themselves in a position to stop you, which means all of a sudden you've created a situation in which you have single-handedly taken out two or three enemies from the fight temporarily, engineering a situation for your team-mates in which they are fighting 4 vs 3 or 4 vs 2. This is the main role of Garen in a big team fight as far as I see him.

In terms of the early to metagame, lane fights, harassment and ganking are the bulk of the combat you'll be participating in, and here Garen is very powerful. In small skirmishes and harassment, don't fret about throwing yourself into the fray, either to get off a crucial Judgment, or to dissuade the enemies from attacking your allies. Hitting either Judgment or Decisive Strike and running towards an opponent will scare them off nine times out of ten, saving your friendlies from a dive or preventing a pursuit. If you're being chased, or are chasing an opponent of similar speed, Judgement can hurt them if they're within the hitbox, which is surprisingly wide. Remember in any case, Garen should not have to return to base for an awfully long time - propelling yourself at the opponents and getting blasted half to death isn't a problem when all you have to do is stay out of range of the enemies for a short period of time and Perseverance will see you through. Remember to prioritise your silence on the enemy with the most powerful abilities to turn an even fight to a 2 vs 1 very quickly.

Summoner Spell choices, from best to worst as Garen, from my experience at least -
Note that if you do choose other summoner spells, make sure to replace the mastery points in Ghost and/or Exhaust accordingly:

Ghost is the most important as Garen, for it allows him to be exactly where he needs to be, when he needs to be there in conjunction with Decisive Strike. This ranges from getting right into the fight from a staggering distance away as soon as either team initiates, chasing down faster champions to apply the Frozen Mallet effect, to simply "booking it" the hell out of there if your team crumbles around you and it becomes a 1 vs 5. I tend to save it for the latter, preferring to use solely Decisive Strike for mobility unless there is a considerable distance to be covered.

Exhaust is a must have in every team - popping this on fleeing enemies is useful, but not nearly as much as putting it straight on the enemy team's carry as soon as you get in the middle of them - not only will this let you survive that much longer, but it will aid the entire team by effectively neutralising a high-priority target until the team can gang up and bring them down.

Heal is a remarkably useful ability - it may seem newbie to a lot of people, but I used this as Garen for a long time, with exceptional results. It allows him to stay in the thick of the action nearly indefinitely - by the time the enemy has brought you down the fight will be long over. It can also help save a fleeing ally, or keep you healthy throughout a tower dive. Garen's passive, Perseverance, means you should only ever need to use this ability in combat, keeping it in reserve for when it's needed most.

Cleanse will allow you to shrug off enemy Crowd Control and show effects to chase down weakened enemies or escape when Judgment is on cooldown, which it usually will be if you've been using it enough.

Ignite is a spell which each team needs at least one of - it helps neutralise the effectiveness of enemy spam healers, such as Vladmir, Sona and Alistar. Popping this in the middle of the fight as they begin to rely on their heals can severely reduce the survivability of these characters. It will also allow you to finish enemies who survive Demacian Justice at range.

Flash will allow Garen the one thing he cannot do under normal circumstances - to cross terrain, giving him an easier escape or chase route. It also allows him to flash on to an enemy and begin using Judgment immediately, giving them no opportunity to run when they see you moving to engage.

Teleport will allow Garen extreme lane flexibility - allowing him to effectively stop enemy pushes in their tracks with the use of a well placed Judgment, which should eradicate a wave of minions easily, as well as letting him quickly switch from pushing a side lane to being right in the epicentre of a team fight.

Fortify is another one of those "you need one on the team" spells - it can allow you to stop enemy pushes dead in their tracks, even from beyond the grave, giving you or your team enough time to get to there and clean it up. Not as useful on Garen, due to his mobility allowing him to be in any lane he needs to be pretty shortly, but useful nonetheless.

Clairvoyance is essential in any ranked team, giving you scouting information allowing for cross-map skillshots, like Ezreal's Trueshot Barrage, or just preventing the enemies from hiding in a bush, preparing for a gank. However, Garen is a primary combat champion, and needs summoner spells to suit - Clairvoyance is something best suited to a support character.

You shouldn't be rolling Revive really - Garen shouldn't be doing much of the dying if he's being played right, using his mobility as a tool for escaping, and there is seldom a situation in which Garen reviving will be able to help - normally, if you've died your team has been aced, and you wont be able to deadstop the enemy team's push by yourself.

Rally is generally a pretty poor ability for Garen to have in his arsenal - the increase in Judgment damage from it will be negligible, as will be the increased basic AD since Garen's attack is so slow, and it is much less useful than other abilities that can support the team, such as Exhaust and Heal.

Smite is a nearly useless ability on Garen, since you would never rightfully play Garen as a full jungle champion, and since Judgment and Decisive Strike will clean up jungle mobs anyway, it's only real use is against enemy summoned units, such as Annie's Tibbers.

I'm not even going to bother with Clarity.

As I've said beforehand, this is just the build that I've had most success with, and I can't guarantee that it will work well with every team setup and everyone's playstyle, but I wish you all the best of luck, and I hope you have as much fun with this Garen as I do!

This is my first guide, and I appreciate any comments and constructive criticism - any sections you would like me to go through that you feel I missed, or any strategies you would have me underline for you.

27/5/11 - First Release
28/5/11 - Added Updates section, edited Items section to fix a punctuation error and to clarify use of "snowball items", added Tactics section
31/5/11 - Edited Items section to include methods of countering Madred's Bloodrazor, edited Runes section to clarify choice of Fortitude over Vitality. Not finished yet, not currently happy with placement of the latter section, will edit shortly.
3/6/11 - No longer requires comment to vote
20/6/11 - Edited Runes section and Rune choices to allow for better survivability in the metagame and endgame. Looking for ideas as to how to manage old runes section, whether to leave it be as reasoning against it, or delete it.
27/6/11 - Edited Items section to include explanation as to my omission of Phantom Dancer from this build. Plan to add to this sections including items such as Youmuu's Ghostblade etc.