Hello, this is my first time writing a build, although I've been using Mobafire for a long time for learning champs playstyles and builds, so go a little easy on me. This is the first time I believe that I can write an effective build of my own design for a new champ. This is essentially a Off-tank near full-tank build that utilizes Yorick's amazing slows and distraction abilities to feed your team.

Pros:
Great pusher.
Can easily hold lanes and solo.
Great burst damage.
Crowds team fights.
Amazing chaser.

Cons:
Moderately to highly mana dependent throughout game.
Needs to farm.
Doesn't get amazing damage without heavy ad items.

Summoner spells I prefer:
Fortify: Great for anti push/dive/backdoor.
Ghost: Best for escaping and chasing.
Flash: Good if you prefer it over ghost.

Other possible spells would include:
Heal: good for tricking enemies into over extending or saving teammates.
Teleport: also good for anti push or for pushing.

For runes, I take flat health quints for the early game survivability, his base health feels really low to me. I find the armor dodge and magic resist marks, seals, and glyphs to be rather self explanatory. I avoid armor penetration as I prefer to let my carry do most of the damage and typically don't need any extra myself, and would prefer to be tankier.

For masteries I take 0/21/9, making sure to get the evasion and nimbleness, maxing my dodge and increasing my movement speed in fights.

Omen of war. The reason for the trinity force. This move has Yorick's best damage potential, and the movement speed buff makes him chase amazingly.

Omen of pestilence. My personal favorite, with a great initiating and continuing slow, this move is what makes my lane. I max this first as its early game slows have the best potential to net my team kills and I found that it often times extends through flash and other summoner spells, where other slows fail.

Omen of famine. This move, combined with the right mana items, allow you to suppress whoever you lane against without being in danger with low health or having to back multiple times.

Omen of Death. Yorick's Zilean meets Mordekaiser ult. Great for allowing your carries to keep up dps in teamfights, allow that mage one last burst, or just to use the ghost for extra damage.

Playing Yorick, I've found him to be exceptionally focus-able, meaning he dies alot, and fast. Building dps just doesn't work for me, as I either don't have the mana to support my damage, or vice-versa. He needs mana items, and even so he doesn't have any spammable skills such as Ezreal or Urgot to justify a manamune. This requires you to take away an ad item in exchange for mana, which seems to detriment him alot. Without early game items, he is hard pressed to get kills as he doesn't have the finishing power of other champs despite his high damage output. Not having any self buffs for attack damage or attack speed, and no hard crowd control (stuns and such), like most other good carries, you're lacking early game damage needed to be fed, and can't make up for and missing damage items if you hope to carry. Thus, you begin to build pure damage and hope you can deal with your lack of mana. Doing so you will quickly find that you have to avoid alot of fights if you hope to not die every time. Because of this you can only spam skills for damage, and then we come back to the lack of mana. Hes nearly useless at this point.
The key to Yorick's damage capabilities is not damage items, but tank ones. When you build this much health, armor, and mag resist, you are allowed to keep a constant presence with not only yourself, but your ghouls as well. Each of your ghouls does a third of your max ad, and has a third of your max health. When all three ghouls are active, you more than double your attack damage through them. You want to be able to keep them on the field as much as possible, which gives you the best overall dps. Building ad items results in ghouls that die as soon as they spawn, making him a burst champion, and not a very good one. Building health results in ghouls that have enough health to land a few hits, thusly they do more damage than when you build ad.
I play him as a tanky support character, and hes one of the best I've played. He has the damage capabilities of Garen, Nasus, or Jarvan, with one key difference. He lacks finishing power. Playing Garen or Nasus, it can be hard to NOT get kills. Characters like these have great supporting abilities, but often end up taking a large portion of the kills, which hurts your team late game. With Yorick, you can still do massive damage, and still hold people down, but do so without taking the kills nearly as much.
Early game:
Take a Doran's shield to start with. You want to take a lane with a ranged champion. Ive had the best experiences with champs such as Teemo and Miss Fortune, you will be leveling your slow, and they can support it best by dishing out heavy damage throughout your 7 seconds of slow. Enemies against this combo lose very quickly as a tanky Yorick provides a wall to absorb damage while keeping the ranged carry farming and fed. Yorick's mix of ranged pokes and tankiness makes it very easy to suppress the enemy lane and keep them under-leveled. At around 2000 gold you want to go back and get your sheen along with atleast part of your boots. With your next chunk of gold you want to get your catalyst and begin work on the rest of trinity.
Mid Game:
Hopefully by now you've fed your lanemate enough that the game will be set, your goal at this point is to have as many assists as possible. With the onset of teamfights, you want to start taking larger chunks of the damage, initiating along with your tank and using your slow to ensure kills. Remember to not use your ultimate on yourself, but the ad carry if they're safe, or your mage if they're in danger. Making a copy of the ad champ gives the most damage with the ghost, but allowing your mage to come back for a burst is more important. Ending this phase you should have your core build of boots, either Mercuries treads or ninja tabi, trinity force, a banshees veil and a sunfire cape.
Late Game:
At this point you should have enough damage with your skills maxed and trinity force built to begin picking up more kills. Your carries should be fed enough to support themselves and you should continue to make yourself tankier to offset the enemy teams carries. You should continue to have enough damage to pull focus away from your carries while taking enough to stay alive.

This build is meant to allow Yorick to tank as much as possible, while still bringing out his damaging capabilities, this play-style allows you to feed your team like no other champ can and although you still need a dedicated tank with hard crowd control, Yorick fills the in where the tank can't and supports the team exceptionally well. I will be updating this as I learn more about the champ.