Marks - Armor pen
Seals - Mana regen at level 18
Blue - +24 magic resist at 18 (+13 is fine too)
Quints - +Attack damage
Magic resist quints, or even health quints are also just fine.
1/21/8 .. For exhaust
I strongly prefer defensive masteries to go with his passive which makes him extremely hard to kill.
I feel that he does not need the offense since he does not land that many auto attacks in the lane and in team fights, and the defensive masteries really help surviving early ganks.
Exhaust and Flash, I personally prefer exhaust over ignite as I feel Yorick wins the lane by wearing/chasing down people and not so much bursting them.
Boots or regrowth pendant, early philosophers stone.
Together with the mana regen from the runes this gives yorick almost all the mana he needs this is why I do not build a manamune
Highest priority - Boots and philosophers stone
High priority - CDR boots, glacial shroud or spirit visage
getting all 3 CDR items is overkill, and I am not entirely sure which 2 i prefer most, CDR boots + visage would rock but leaves him short mana in the lane to harras early.
Shroud + visage sounds solid together with tabi or mercs, I havent actually tried this combo yet but I will report my findings when I do.
Mid priority - Atmas + aegis and/or warmogs
Time to get some damage, atmas is in my opinion almost the only damage item yorick needs, it works really well with building very tanky on him, I don't worry about slows or speed increasing items, with enough CDR he gets very mobile between his q and w.