Trundle is a very special champion. Selflessly, he has taken it upon himself to carry the burdensome curse of his entire tribe, so that his troll-folk may live on in peace and happiness. Meanwhile, he is hiding in that bush outside your lane, waiting to share with you the love he holds for his people. This love is his club, and Trundle believes that his club deserves all the romancing and flirting that he was denied in his Troll-teen years. So here he is, arms ready to envelope you in warmth, and club prepared to practice kissing your face...

Actually, Trundle doesn't care about you. He just wants to beat you to death. I want you to understand that.

But seriously, Trundle is an exceptional champ. He makes a great jungler to have on your team - so long as he is played well. He cries out for two roles in my mind: Ganking and Tank-killing. He is a solid, melee off-tank champ with great ganking skills, and good skills to weaken the annoying enemies. He will out run almost any other champ to escape or close that distance. When he decides to turn around and hit someone, chances are they'll feel it... keenly. (Yes I said 'keen'. Don't judge.)

Masteries:
I go with a 1/19/10 mastery build which I have found to work really well.
In the offensive tree I immediately go for Smite, because Trundle belongs in the forest (didn't want to rhyme). His power is just more noticeable with ganking opportunities.
The defensive tree has many points put into it. Being in the jungle, it's best to take less damage so that you can keep up on your jungling sequence, and don't always have to go back to heal before you attempt a gank.
The ten in utility are for the basic jungling masteries which, in my humble opinion, are a must.
If you don't go jungling, you could rearrange this with fewer points in utility and more in offense. Taking exhaust as opposed to smite is ideal if you're laning.

Marks are offensive by nature. As Trundle is an Attack Damage champ of the DPS persuasion, Armor Penetration is a must.
Seals give us a defensive opportunity, and Armor is what we need the most for survival in the jungle.
Glyphs provide better utility than defense, but Magic Resistance per level helps us compete slightly later in the game.

Start with a Vampiric Scepter, please. It's harder than Cloth Armor and 5 potions, but it's much more efficient (it's majorly helpful if you can get someone to leash or hit first and run away). If you can kill blue golem with a V.Scepter and not use potions, that's money in the pocket. It can be risky, but it pays well.
Boots of speed and a Wriggles lantern should be rushed. Wriggles gives you map awareness that, without otherwise buying wards, you need but just don't have.

Sheen is a must, as it procs with your Q. A spammable auto-attack enhancing ability makes Trundle viable. It's great.

Mercury Treads are the best boots for Trundle. When stacked with Contaminate, your CC reduction is really quite disgusting. If, however, the enemies don't have any significant CC, go with other boots that you fancy.

The Brutalizer straight into Youmuu's Ghostblade. The early Armor Penetration is gold for us in the form of effective ganking. Youmuu's itself is one of my favorite items for Trundle. Imagine: Ghostwalk > Contaminate > Ghostblade chasing or escaping. It's a sick movement speed combo which will give you a major advantage.

At this point, I have listed: Trinity Force, Guardian Angel, and Tiamat. Don't buy in that order unless you're prepared to. This section is more of the 'After Core Build' part. Chances are that you need a defensive item. Whether it's GA or not is up to you. Trinity Force is pretty paramount to a good Trundle late game, so it's a core item, but a later buy. Tiamat is just cool. I love it on Trundle, but depending on your match, buy it, or buy another defensive item, or buy a more appropriate damaging item. The match shouldn't have gone on this long anyway.

My favorite items on Trundle sometimes include:
Spirit Visage (which affects his passive)
The Bloodthirster because it's a great way to have survivability and damage late game so you don't fall behind so much. This should really be where Tiamat is listed, but I like the splash.
Frozen Mallet is great so you can skip getting Red and give it to an ally instead.

Rabid Bite is the point to Trundle... Hurt someone badly, and make them not hurt you. It's simple really. Max this first.

Contaminate is one of two of your ganking and escaping skills. Max it second. Use this if you need to move, or clear a neutral creep camp faster. I use it between camps all the time early on... Careful though, it can make run out of mana sooner rather than later.

Pillar of Filth requires only one point early on to be as good as it needs to be. The slow is helpful, sure, but that little wall is AMAZING. Use it in every gank and in some harass. Place it directly behind or in front of the enemy to block of the direction you do not want them to run. Place it against a wall so they have to come closer to you to escape. I've blocked a person against a wall and trapped them with just Pillar. It's a sick ability that's useful anywhere. Block the enemy, block your allies from going into a stupid fight... You know Ashe's arrow is coming for your stunned or snared friend? Place it right on top of them to force them to move and be missed by that annoying arrow. That's right: Pillar of Filth will forcible move anyone standing on the spot it rises from. So cool.

Agony is one of my favorite ultimate abilities. Steal health, armor, and magic resistance. Save this for either the tank, or the person you want to disappear. It's a killer in a 1v1, and it lets you solo dragon relatively early.

Ghost and Smite are the best for jungling. Smite makes you more efficient and much quicker in the woods, and it provides the opportunity to steal dragon or Baron. Ghost is your ganking skill, but since you do have Contaminate (and probably Ghostblade), you may not need it.

Alternatives are:
Exhaust. The best option outside of smite. It makes you better at 1v1... A lot better.
Flash is an interesting choice. Gives you many escape and ganking routes. And movement over walls is a great escape.

All the other Summoner Spells don't really go the distance for me, but may work better depending on your allies or enemies.

Jungling is where this Troll belongs. He has an escape and a chase in two abilities! This makes him great for ganking. He could go bottom lane, but not mid or top so much. Bottom lane puts him near dragon, and he's well suited to help the jungler kill it.

The route is the common one:
Begin with Blue Golem. Hopefully an ally can smack it for you once or twice. Smite to be sure.
Move to the wolves and smack them on the nose.
Move on to the wraith camp and focus the big one. Get that damage off of you sooner, not later.
Make a choice: you should be level 3, when you kill Lizard, you will be 4 and can begin your ganking (you'll get pillar), but you're low health. Kill lizard and minions now for extra money and then go back to buy and heal then come for a gank. -OR- Heal and buy first, then get lizard and golems so that you will be out there using red buff for longer (no time wasted at shop with red buff). You decide.
Kill Red Lizard.
Destroy Golem camp or make a gank first on one of the side lanes.
Move to mid and attempt a gank.
Run around trying for ganks and get kills in camps along the way or until the opportunity for a kill presents itself.

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Another path is to start at wraiths or wolves, but I'm unfamiliar with that. It's safer, but not quite as effective. It still works, so google or find it on another champ's jungling guide.

Trundle is a Pro:
-Strong against tanks.
-Great jungler.
-Great ganker.
-Survivability is very high.
-His Q is an offensive way to defend yourself. It's effectively damage reduction!
-He has a wall that can't be passed through. Better than a slow.
-His ultimate weakens an enemy, and strengthens you.

Trundle is a Troll:
-No real ranged attacks to speak of.
-No awesome skins.
-Poor laner (easy to get harassed).
-Passive isn't exactly amazing.
-Starts to fall behind in damage late game.