I originally built this guide for a buddy of mine but i figured i may as well post it and help everybody out. Yorick tends to be seriously underestimated, he can also take awhile to get used to and should be played by people with a decent amount of experience with the game.

I play yorick as a very tanky AD champ.
once you manage to get the hang of how yorick is played you can easily dominate your lane and beat the s*** out of your opponents

having said that enjoy the guide and feel free to leave some comments/upvote/criticize

Pros-

awsome lane bully
can easily solo
omens are awsome
early game BOSS
Picks up late game
awsome skin
becomes very tanky

cons-

tends to get focused alot
unless you can farm well, becomes difficult to keep up in damage mid game
uses a shovel...

Yorick is an excelent bruiser, especially early game. The flat armor seals and flat MR glyphs boost your survivability and make you a serious threat early game. The Armor Pen. Marks and quints, are there for obvious reasons, so that when yorick hits someone , they feel it.
Greater Mark of Desolation and Greater Quintessence of Desolation

So, as I explained earlier the armor penetration that you get from these adds up to 25% armor pen. From experimenting with a couple different marks/ quint combos i found that this is the most benificial. It has been suggested to me that or would be better suited for Yorick. My counter argument would be this situation: if you hit someone with an unecessary amount of armor with just a little extra damage or just a few extra times, it wont really get the job done as well as if you just ignored a portion of their armor.

Greater Seal of Resiliance and Greater Glyph of Magic Resist

now, why flat armor and magic resist runes instead of armor and magic resist per level from Greater seal of defense and ? This is a matter of preference to the player, however I find that with the suggested build, has enough armor and magic resist to survive most encounters and doesnt benefit as much late game from these runes as he does early game with flat armmor/ magic resist.
however, like i said it is a matter of preference to each player to decide what the like better or find more useful.

Im gonna keep this short and simple

21/0/9 mastery setup

Offense:

the primary points you really wanna hit here are the armor penetration parts combined with the runes, your gonna come up with about 40% armor Pen.

Defense:

Personally I find that Yorick benefits much more from the other 2 trees and with this build, has enough armor and health to survive all but the toughest of fights.

Utility:

The key points to hit here are the increased mana pool and Mana Regen. Early game you wont have your insanly large mana pool to rely on so your gonna be happy that you put a few points into this Mastery tree. primarily becuase you spam yoricks omen's, making him very mana hungry early game.

The key item in this build it . This is the source of most of yoricks damage. start off with the [iocn = Meki Pendant size = 35] and 2 Heathl Potions, wich will help you out with your ever dwindeling mana and keep you in lane longer. next come the then your , Finish your mercury Treads or (depending on if its an ap or ad heavy team) and finally your Manamune. By now everyone is ****ping their pants and running from your shovel which all of sudden seems to hurt alot more.

This is an important part right here. go for the now, its armor boost will help your suvivability and the extra mana will not only boost your pool, but consequentally boost your attack damage because of the manumune. next comes the Catalyst the protector. you may be asking "But turtels, why not finish the frozen heart first?" well my friend right about now, you would be noticing your health is rather low, this solves that problem as well as boosting your mana/damage.

next is where you can vary depending on your situation, if your facing a AP heavy team, go for the . if its an AD heavy team go for . either way you should end up with both of them.

now if you can manage it start your , go for first and try to proc its passive with your omen of war, esentially nuking your target. once you finish your Trinity Force(at which time i congratulate you on getting this far in the build) now finish it up with the , going into the most awsomely named item yet maw of malmortis.

This whole build leaves you with a massive mana pool, pretty solid health, alot of Armor and MR (making you VERY VERY hard to kill) 2 shields (Banshees and shield from the Malmortis) a ton of attack damage (I finished with just over 300 at the end of a game) and decent attack speed.

Unholy Covenant This is something youve got to remember when your fighting, the more omens that youve got running about, the less damage your gonna take overall. So try to keep as many as you can ready for a team fight. or if your about to get hit, just send out your omen of pestilance since its got AoE and doesnt need a target to be activated.

I usally start out with the , the extra boost in your swing does some serious damage. on top of that the extra movement speed is a real help for hit&run gameplay. This is the skill to max out first.

next comes the . This is the skill thats gonna keep you going with its sustain. its also a point-and-click style skill with a decent range for a melee charecter.

*note- if your more of a stand back and harass kinda person early game, your new and dont really have masteries/runes/experience, or are solo laning, you can switch the skill sequence between the Omen of War and the Omen of Famine.

I save the omen of pestilance until the end, only putting one point in it for the beginning, it really doesnt have that much damage, especially since it scales off of AP instead of Ad like the other 2 omens. Its good for slowing but thats about it.

your ult, Omen of Death is pretty unique, its like a combo of and ult. The ally champion you use it on will have a copy of them made. If the champion is still alive, the copy will act like Annie's Tibbers. if the champion dies, they gain control over your omen of them until either the omen dies, or its timer runs out. Note that the damage done by the omen is based off of a percentage of the original charecters damage. Its incredibly useful for either giving you ally a little more time to woop a** or to create a temporary team of 6...

Yorick works well with both a team and by solo. what many players seem to conviniently forget when fighting Yorick, is that the more omens he has out, the harder he is to kill. To begin with, a charecter with over 150 armor and MR is hard to kill, but when he takes 35% reduced damage on top of that, it makes him far harder to fight.

aside from the reduced damage, spamming yorick's omens can take a serious toll on an opponent. The omen of war can easily do 300-400 damage a pop, which can be done every few seconds, the omen of famine gives you a nice little boost to your health as well as some damage, and the omen of pestilance will stop them from fleeing when they realize theyre about to lose that fight. if things start going downhill, pop your ult and give yourself an extra few seconds to get that last kill and win the fight.

when with a team, its a good idea to initiate a fight. yorick is tanky enough so that he can survive all but the hardest of onslaughts. if your nervous about going into one, pop your ult right then and there to save yourself time and make an additional target.

5/2/2012 - BB coding added, rework and update of sections.
special thanks to "moderatpro" for suggesting it and to "jhoijhoi"
( http://www.mobafire.com/league-of-legends/build/making-a-guide-101506 )