Over the past week I have been playing Shen, and from what I have seen, he can build very tanky most of the game and still put out decent damage up until mid-game is over. Nearing end-game he is never focused, therefore giving him a reason to get a DPS item.

I geared this build/guide more towards a dominant early game so farming is easy, mainly because many times your game is pretty dependant on the first 15 minutes when the Doran's shield and straight defence runes help the most.

These runes are very important for this guide to work.

I put more armor on this character build simply because when a tank initiates, carry and tanky DPS champions will usually get a few hits on him, while casters that do magic damage will save their cooldowns for later in the fight to take out less tanky allies. After the initiation you really shouldn't take much damage unless your team lost the fight, in which case you'll die anyways.

Please read the whole guide before you criticize or comment, maybe you'll understand something from my explanations, or you can find a counter-arguement and teach me something.

My rune choice came down to what would help him end-game, but make him strong right at the start.

Marks - Straight MR because the MR/level is terrible for on marks, and he doesn't need offensive runes to put out damage.
Seals - Straight armor to make him dominant early game, and be allowed to harass while taking little damage from minions.
Glyphs - I went MR/level because it scales well per level and this build has more armor items, so it helps end-game.
Quints - Again, to have a strong early game and to shrug off harass.

I don't think the masteries are as important on this build, they're more geared towards play style in my opinion. I chose these masteries because they make minions do next to no damage early game while also providing the 4% damage reduction, CDR, free spell, and a buff to exhaust.

-Original Build-
I start with a Doran's Shield for the hp/5 and armor along with the nice chunk of health that helps the passive.
Next I either go for an early Heart of Gold to help with money, or if I am getting hit hard by magic attacks in the lane, I get my Mercury Treads first and then a Heart of Gold.(be sure to get at least speed 1 boots before or right after your heart)
Then I get a Giant's Belt to back up all those resists I have. If their team is magic damage heavy, I will get a Negatron Cloak before finishing up my Sunfire Cape. After the Sunfire Cape I finish the Force of Nature, which means you never have to go back now (if you even still had to).
Next I get a relatively cheap Thornmail (I usually sell Doran's Shield to get the chain vest, but you can just outright buy the Thornmail and keep the Doran's Shield for now.) to make diving easier, and to deter ranged carries from fighting back and lifestealing over my damage.
Then IF the game lasts this long, damage is needed because the Ki Strikes and Vorpal Blades just aren't cutting it anymore. This is where the Trinity Force comes in, usually the game won't last long enough, but when it does, and you are tanky enough that the enemy team will avoid you completely until everyone is dead. Instead of sitting there doing nothing with your pathetic damage, why not tear up their squishies? A Trinity Force increases damage by an insane amount, especially with the high frequency that you can throw Vorpal Blades. Usually, I get a Sheen as my last item slot and then save for the 2,800ish gold to upgrade to the Trinity Force.
Finish Randuin's whenever the opportunity is given, especially if their physical DPS is tearing up your team.
-Alternatives-
Trinity Force can be replaced with Rylai's Scepter for a slow, a stronger ultimate, more health, and more damage. Trinity Force in comparison is a few hundred health less, a bit weaker of an ultimate, but increases move speed, and has way more damage.
Thornmail could be replaced by Guardian Angel, but I find it too expensive for less stats and a weak passive (before the nerf it was amazing/OP). It could also replace Trinity Force if you just want to be tanky and run in there.
Atma's could replace Thornmail and make a Phantom Dancer a viable replacement for Trinity Force to make you very scary to all squishies.
Any item can be taken out if their team has underwhelming damage against a resistance and be replaced with an Aegis of the Legion which could help you rush a DPS item.

Vorpal Blade leveling is a must, it helps you to stay in lane instead of backing, and also to farm and harass. At least one rank of Shen's taunt is required to either escape, chase, or even finish an enemy off. Taunt also has a reduced energy cost at higher ranks, whereas Shen's shield has the same at every rank.

In my opinion, skill importance goes as follows:
Stand United(R) > Vorpal Blade(Q) > Shadow Dash(E) > Feint(W)

Flash is yet another ability you can use to escape or secure a kill, or even to get to a teamate so you can taunt away the enemy attacking him or her. Flash helps reduce your deaths so you don't fall behind in experience and it helps prevent feeding.
Exhaust helps early game to prevent a kill or to secure a kill, and later into the game it can save a teamate when a high DPS member of the enemy team unloads on them.

Most summoner spells can be substituted for these, but I find flash the most useful and diverse summoner spell.

Suggested Summoner Spells:
Ignite
Heal
Ghost
Clairvoyance
Fortify
-For these summoner spells, you can tweak your masteries to pick up specific perks besides the one for Clairvoyance.

Laning:
Save Ki Strike for harassing the other team or last hitting, this in combination with Vorpal Blade makes farming really easy while also letting you have a harass ready when needed.
If enemy minions are pushing towards your tower and your minions aren't there yet, and of course if you can spare the health, lead them up or down away from your turret depending on where you are, this allows more last hitting while also keeping your minions from pushing so you can stay safe.
ONLY last hit! The only acception is if you are about to fight and want a bit of a minion damage advantage, or if their minions are building up too much.
Denying experience is hard, and usually not possible, but if it is possible it can really hurt their team member's farming and experience gain while also causing them to choose between overextending and losing experience.

Juking/Assisting:
If you are trying to save a teamate, but are in danger of dying yourself, you can taunt through a wall to buy that teamate time while also staying relatively safe. (The best way is if you are at a point where you can taunt so pathing takes the enemy away from your teamate, but either way, a taunt helps.)

Teamwork:
Announce to your team when you hit level six, try to pay attention to your team on the minimap, and ask your team to ping if they are fighting because you can't catch every single skirmish throughout the game while maintaining lane dominance.

Dragon:
At about level ten you should be able to solo dragon without much trouble if you picked up the armor items, if dragon is on your team's side, keep your energy reserves high enough to dash through into the jungle. If dragon is on the enemy team's side, pull it out far enough so that you can see if an enemy is coming (unless you have a ward, which you should, in this case you should plan an escape).

I had a few hours of extra time, and put some of that into this guide, it isn't top quality, and I'm not done, but hopefully some of you can get something out of it.

If I actually get feedback I will update this guide and actually put work into it.