Welcome to my Shen guide. I recognized, that there is no adequate guide to the way I play Shen, and since my build works pretty well, I'd like to share it with you.
This build includes the standard build for dual lane, a slightly different one for solo lane and some jungling tips.
The build starts with some not-critical stuff like explanations for my choices of Runes a.s.o., so if you just want to trust me and rush into it, start with chapter "How to play" and follow the cheat sheet.

• Shen is definitely the best support-tank in League of Legends.
• He is one of the characters with most HP.
• The only char with taunt who doesn't get banned in every ranked.
• Shen has ultimate map control when using teleport.

Pros

• hardly killable from mid-game on
• deciding the team-fights
• ultimate help in every bad situation

Cons

• not as resistant in early game as other builds
• no kills for you

It is obvious that a tank needs to skill through the Defense Masteries tree. Although there are ways to play Shen as caster, ad, and even tank, caster or ad as a jungler, we will focus on Shen's main target: Being a tank.
(The two skillpoints on Defensive Mastery may confuse you, but there are many situations, when you just get a huge amount of dmg from creeps, considering them as no problem for you.)

More debatable is the distribution of the remaining 9 points. But as you might remember you are a tank, so the Utility tree with it's lifereg, xp and gold seems at least more fitting that critchance or ap.

Marks

9 x Being a tank you need to stack armour and this is the best way to get it.

Seals

9 x Combined with your Defense skills dodge is a very successfull way of avoiding high ad dmg and espacially running away from it.

Glyphs

9 x Same as armor from Marks, but even more important since you cannot dodge spells!

Quintessences

2 x Although these Quintessences were nerfed some time ago they are still the base of all early-game surviving.

1 x Greater Quintessence of Vitality Sure you want to survive the early-game, but we've done enough for that. This Quintessence will stack with your Warmorg's to give you an enourmous lifepool in late-game.

Spells to get

For me teleport is the key to map control. It's okay with normal heroes, but it makes Shen appear freaking everywhere.
Combined with it allows you to escape hopeless situations. Especially when pushing alone you will need to save your butt a lot of times. Not to forget the chance of terminating a chase in 1 second.

Good skill to travel fast, as escape skill slightly behind Flash due to the combo. Good for chasing over longer times, too.
Make Shen the ultimate supporter! Mostly useful, but rarely used because you have to replace either the escape or the map control spell.

Not to get

Everyone can pick ignite, why should you waste your slot for it? Let the others get the kills.
I see many tanks walk around with it and in fact it is a nice spell in some way. But 500 life won't save you as much as a blink could do.
Dude, seriously...

For me Shen is the best support-tank in the game. He is NEITHER under the top 10 heroes with most life NOR with most armor/MR, although I read this in some guides. So how can he be the best support-tank in game? You make him being that!
What you need is first being a tank. What have we done so far for that: Runes and Masteries.
Now it's time to move on.

What we want is to support and tank. What we don't want is deal dmg. So it's self-explanating, not to skill through. But you do need it at lvl 1 to get a bit of farm, what generally is not easy to get with Shen. You CAN skill it up again once at lvl 3 if you play solo-lane.
At lvl 2 you get , because on one hand you can save your mate's ***, on the other hand you can get your enemies attacked by towers, if you let them push and taunt them in tower range, what you really have to do. At lvl 3 you get ,just so you have it.
is the ability you need to level up first. Except your ultimate this is the only way to support your team. 2nd priority should lie on , because it makes you even more tankish.

Why not level up the blade?
The blade is nice for jungling and farming. So if you play single lane, I would recommend to skill it to level 2. But all in all DMG is not what you want to do. Your taunt is one of the best CC in the game. (And the blade even got nerfed in patch 1.0.0.118...)

As it is for any tank, the items are very situationally. Anyway, there's a core you almost always need to buy:

Boots:

As Shen you can choose between these two boots:
Normally I buy these. They bring you up to 18% dodge. Combined with your Masteries you will get increased movement after every 5th hit you take, also from creeps. If you are not sure, which boots you should buy, buy these.
In case your enemy team has much CC and/or AP chars, the Ninja Tabi would lose their sense. Although Mercury's are expensive, you will be rewarded for buying them in some cases.

Situational Items

Buy this item, if you get too much magic dmg. You can buy it after or even before it, depending on your enemy team. If you don't need it at that points of time think of it as your last item maybe. If you still don't need it there, you should be unkillable anyway.
If you don't have to, don't buy it. The mana you would get is lost money. Buf if there's for example a Nocturne in the other team it can block his ultimate and you can save ganked teammates, that wouldn't have a bit of a chance without you. Also useful against a fed Karthus etc.
This is THE item against AD. 100 armor are sick and you give them 30% of their dmg back. So if there's too much AD: buy it!
Also an anti AD item. But it also gives life and dmg around you, so it's a bit more universal than . If there's good AD from your opponents it is almost always a good idea. When there is outragious AD you can combine these two items to make you unkillable against AD and by the way deal big dmg to them.

This is Shen's passive. It makes you perform a powerful hit with bonusdmg depending on your hp. After patch 1.0.0.118 the basic dmg is reduced and you hp give you way more dmg. (That's one reason why we choose as first tank item.)
You throw your blade at an enemy, dealing some dmg and giving attackers lifereg. Perfect for lasthitting and jungling, useless for tanking. (That's why we choose Vorpal Blade as first skill.) The dmg at higher skill levels even got reduced in patch 1.0.0.118.
A perfect tank spell. Gives you a shield for 2 seconds that absorbs some dmg. It is not really mighty unless you stack AP, what we won't, but still helps you to survive way longer.
You dash forward in the appointed direction, taunting all enemies on your way (that means they have to attack you). Shen is one of three Champions that can taunt (Rammus, Galio) what makes you very valuable for your team. In late game, when you almost take no dmg anyway, it is kind of a 2 second group stun.
Your mightiest spell. You shield an allied and teleport towards him. But care: you don't shield yourself.

This may be the most important section. Please read it carefully if you want to succeed with this build.

Early Game

You start your game with a . Instead of going to you lane now, you wait at you base until you got 35g for a health pot and then instantly to your lane. You won't lose any XP.
The early laning phase you will normally be pretty passive. You skilled your and try to lasthit every creep with it. Don't waste it on your enemies, it really does no dmg. Completely different is your . Perfect for lasthitting, but it also really makes good dmg at enemy champs. Try to get verly little dmg all the time. Only from lvl 2 on you can taunt your enemies and let your lane mate deal some dmg. If you're sololane don't try to hit them except they come to your tower range. Then taunt them and see if it works. You can also skill to lvl 2 at sololanes.
You can also use to dodge enemy spells or escape from sticky situations.
You will go back to the shop as soon ans you have to, but try to wait until is ready.
Now you have to decide what boots you will take. Look at the Item section for this.

Mid Game

You are on your way to build you now and are beyond lvl 6. Your eyes should now be 50% on your lane and 50% on the map. As soon as you see a fight on the map, place yourself in a safe position (or let Shen push by himself, if there's no enemy on your lane). If needed use your ultimate to save some teammates. Due to teleport you can be the one that controls the map by saving teammates and towers, but also push towers.
Your next item should definitely be . It makes your support function more effective, gives you HP, MR and Armor and has a nice effect on your friends.
For the rest of your item build please read the Item section.

Late Game

There are different definations on the point of time when the late game starts. For us as Shen it is the time we will only find ourselves running and teleporting from lane to lane to push or def towers, having hundreds of teamfights, where we are always in the middle of the enemies, taunting them to death, and save teammates from ganks, if it makes sense to ulti on them. Anyway you will have a lot to do, so don't waste your time by pushing lanes alone. You are needed in your team, always look for someone (or something) you can protect.
This is the time I leave you alone with the feeling of being damn important, the reason why it is so much fun to play Shen.

Please don't forget to vote and tell me your opinion on my build.
RoyPJ