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So this is my build for Shen. I should mention that this is my first build I've ever made, and therefore, probably won't be too in depth or detailed, but I'll tell you what I think of Shen, and how I like to play him.
I like to play Shen as a solo-top brusier. His ability to heal with his Q (Vorpal Blade) allows him good health sustain, and since he's based on energy, there really isn't a secondary limitation to his laning ability. With the combination of health sustain and no mana, Shen is great at stying in lane and farming up the gold he needs to best support his team.
Anyway, this is why I like to play Shen:

Pros:
Shen is everywhere. All the time. His Ult along with Teleport make him everywhere all the time
Shen can build AP, and still be tanky with his W (Feint).
Shen can save lives by either blocking damage with his W (Feint), Ulting for a Shield, distracting with his E (Shaddow Dash), or allow for healing with his Q (Vorpal Blade).
Shen's mobility and escape mechanisms with his E (Shaddow Dash) make him quick and difficult to catch, especially for a tank.
Shen is a flippin' ninja. He's got a buff that says so.
Cons:
Not much damage. Shen's AP scaling on his Q (Vorpal Blade) isn't much, and his only real damage abilities are his Q (Vorpal Blade) and his passive (Ki Strike), the latter of which scales off of maximum health, meaning that he has to build tanky to do damage, but in the process, limiting his AP, ergo his damage.
Shen can't really spam as much as you want him to. His energy regen is amazing (50 per 5, like all energy based characters) but his energy is only 200, and doesn't increase, meaning he can spam all his spells, and then can't cast any more for a few seconds, even after cooldowns are off.

My masteries for Shen are a little bit odd. I play him with 9/17/4 masteries. I want to give him good defensive masteries, as he is the tank of the team, but I also want him to actually have power in lane, so I give him some damage increasing masteries, and improved summoner spells (lowering the channel time for teleport, as map mobility is nice on someone who creates kills like Shen, and decreased cooldown for flash) and higher total energy, for more ability to spam his spells when he needs to.

I like to play Shen a little on the agressive side, so I give him Ability Power per level Seals and Glyphs, with Magic Penetration in Marks and Quints.
The AP per level actually square out with flat ability power at about level 6, which means that you're going to be doing better for more of the game, later in the game, since the brusier top usually doesn't get many kills or push many towers, especially in the early game.
The Magic Penetration runes also go with the more offensive build I like on Shen, as the other solo top brusier will most likely have a respectable amount of MR naturally. Although your 15 Magic pen isn't much to start with, the relatively quick Haunting Guise gets you at about 35 Magic pen, which is a little less than the MR most Champions will have, as long as they have the 6 MR from masteries and flat MR Glyphs or even MR per level Glyphs.

My personal favorite summoner spells on Shen are Flash and Teleport.
Flash is everyone's favorite "Get Out of Jail Free Card". Couple Flash with Shen's E (Shaddow Dash), and you can get out of most any sticky situations (presuming you're not an idiot).
Teleport makes it so that the enemy feels that Shen is literally omni-present. If ever there is a fight you're not a part of, simply watch for who is going to get focused and ult if. And if by chance, your ult is not up, or you're not level 6 yet, TP to a nearby minion or ward and join the fray. Being in every fight all the time isn't something your average tank can do, and that's why Shen is special.
Other Summoner Spells include:
Exaust - Not a bad spell. Not as usefull with the AS and AD debuff if you're facing someone like Cho Gath, but a huge slow, and is great in helping with ganks from your jungler.
Heal - Not bad if you are new to Shen or even just new to LoL. A good buffer, and great for baiting enemies into tower diving to destroy a health pool that is higher than they really know.
Ghost - Eh. You already have two escape mechanisms (Shaddow Dash and Flash) so not really necessary, unless you like the gap closer in favor of Flash.
Surge - Actually a legit possiblity. Attack speed allows for your passive to get off more often and AP helps since Shen scales off of it. Better for people like Teemo or Kayle in my opinion, as they run off of damage through AP, but constant applicaiton of that damage through attack speed.
Cleanse - Not a bad spell, really. If you find that you need it, you could just buy a Quick Silver Sash (gives MR and offers Cleanse as a Unique Active). In my opinion, you need Teleport for aiding your team and you could have just Flashed out of any situation that required Cleansing in the first place, but hey, it's your call.
Ignite - If you plan on having an agressive Shen like I do and really dominating your lane, this is actually a very good choice. The extra damage output is nice, and since you'll be facing a tank, they'll have lots of resists, which are ignored by Ignites true damage. So all in all, a pretty good spells for an agressive top lane.
Promote - If you really want to beat that guy up top and take his tower in the early game, this is how to do it. Your now Uber minion will be dealing more damage, quicker, and has more health. This means it can do more damage to the tower (as minions deal more damage to towers) and since it has the sheild from half of the towers damage (since it's still a siege minion) it's extra health will help push that tower all the better. If Promote fits your play style, go for it. Doesn't work for me, but it might work for you.
Smite - Great for jungle Shen, as he needs the extra damage output to be a successful jungler. But we aren't here for a jungle Shen, we're here for a Solo Top Shen, and that's what this guide is about.
Clairity - 'The hell are you thinking?
Revive - If you pick this spell, then by God, sir, you are an idiot. 'Nuff said.

Since you're laning on top, level your Q (Vorpal Blade) first.
Since mana is no limiting factor, go ahead and spam it until your pinky bleeds.
Then comes an early point in E (Shaddow Dash).
Shaddow Dash has great utility. If ever someone is on your tower, force them to attack you with your ability. This will force them to deal small amounts of damage to you while still pulling tower aggro, dealing lots of damage. This also give Shen some energy regen when he hits a champion and it's also a dash, which means it's an escape mechanism. Take one point in it early, but max it last, as nothing about Shaddow Dash improves drasticly with level.
Then max Feint by level 13.
With your moderate amount of AP, you can sheild for a good amount of damage, even at early levels. If you expect that you're going to pull creep aggro, tower aggro or harrassment, activate it. This can also be used offensively, as when Shen attacks a monster or champion, the mandatory 8 second cooldown on his passive is reduced by 1.5 seconds. While Feint's sheild is up, this attack based reduction is doubled, so his passive magic damage occurs more often.
Then of course, there is Shen's Ultimate. This is what makes Shen Shen. This really makes his AP become more useful. the Scaling is 1.5, so with his 200 AP end game, his sheild at level 3 with give 1150 health to an ally. And also you're teleported to the target ally. No biggie.

Shen is Shen, and Shen is awesome. This means he has to have some awesome items to go with his awesomeness.
For Shen, I always start with Doran's Shield. With the nerf of health that came to the other two Doran's items, it makes the 120 health given by Doran's Shield that much better. The health up his survivaility, and also gives damage to his passive and better healing to his Q (Vorpal Blade), the armor is always useful, because in very early laning, everyone is going to be basic attacking, even characters that don't go attack damage have to attack with basics when their 1 or 2 spells are on cooldown. And of course the health regen helps Shen stay in lane through those early levels.
After that, I always take boots. This is acutally what I do on pretty much every character I play. After my initial starting item, I pretty much always go for mobility. Whether jungling, laning or supporting, boots are great for mobility, getting back to lane and dodging skillshots (E.G. Ezreal's entire skillset).
Usually when you go back, you've got enough money for more items. Now usually I get the Amplifying Tome first, and then Cloth Armor (Null Magic Mantle if you prefer), but if you wish to play more agressively in the laneing phase, go ahead and take Haunting Guise and come back to your resists later.
Then you're going to want to change your boots into Mercury Treads. This will give you even resists between the armor Doran's Shield gives you and the MR Mercury Treads gives you. The tenacity is nice for getting out of CC so you can help support your team and soak up more damage so your carries can kick some ***.
After that, I like to get more all around tanky, which is why I love Aegis of the Legion. good amounts of Armor, MR and health, not to mention a small tanky buff to all surrounding allies.
Then get some more damage through a simple Blasting Wand and a Sheen. The mana on Sheen kind of goes to waste, but the next hit damage allong with your passive means that one hit will do some substatial damage and possibly cause an opponent to back off. The total AP from both items is 65, which you can't really complain about.
Then look to turn that Blasting Wand into a Rylai's, because it give you more damage, more health (ergo more damage via your passive), and a nice slow, which helps set up kills for your carries.
By this point in the game your Doran's Shield has served it's purpose and is out shaddowed by your other items, so sell it and buy a Chain Vest that will eventually go into Sunfire Cape, and a Phage that will eventually go into Frozen Mallet.
Finally, look to complete your second tier items and turn them into your late game tank and damage items which are Lich Bane, Sunfire Cape and Frozen Mallet. If you want more damage, MR and movement speed, go for Lich Bane. If you want the magic damage and health, look to finish Sunfire Cape. If you want your enemies permaslowed with a combination of Frozen Mallet and Rylai's, complete Frozen Mallet first.
Either way, at the end of this build, you've got a good amount of resists, lots of extra health, some nice extra CC, and increased attack damage (Via Frozen Mallet and Aegis of the Legion) and a good amount of AP.

You need to remember your role as Shen is one of a tank. You aren't a carry, and you're not a support (Although this is not stricktly true,m since you can block skillshots with your body and W, and your ult can save lives). Your job as a tank is to disrupt the enemy team (Your E is key in doing so) and initiate for your team (also possibly with your E). You need to do your best to attract attention of the enemy team, while still staying alive. This is a difficult role to play, as you must border on the line between tanking and feeding. Remember that you have to be the first one in, and the last one out of any fight. However, you shouldn't over extend, especially when you're low on health. You should also remember that you need to feed your carries. You need to take damage for your squishies, disrupt the battlefield, initiate fights and set up kills for those who will do the most damage. But into the end game, when your carries are fully farmed, and have completed their entier builds, don't fear to steal kills, as you're going to need the gold to actually finish your build, as opposed to wasted gold on your fully built carries.
Another important part in playing Shen is that you need to be everywhere, all the time. If you're past level 6, wait for the enemy team to initiate a fight, then ult on whoever is being focused to midigate damage and enter the fight quickly and start tanking and disrupting. If a fight with 2 or more people occurs before level 6, use Teleport to get to a nearby ward or minion and join up and help your your team. as you have some early AP and health, you'll be plenty tanky, while still doing respectable damage.
Just remember your role as Shen and you'll do just fine.

In the laning phase, remember that you have a nice ranged poke (Vorpal Blade). This paired with Shen's energy means that poking should be happening all the time. If you ever want to go in and really get off alot of damage, activate Feint, use Shadow Dash to initiate, get off your passive with a basic attack and either zone out your opponent or just run back.
Remember that you can last hit from range with Vorpal Blade, and you can kill minions with larger health pools by basic attacking with your passive up. While your late game farming power is pretty bad when compared to even some support champions (E.G. Zillean w/ Time Bomb), your ability to farm in the early and mid game is pretty good.