So, um here's a mini guide for those who don't know what to do with AP Mids

AP Mids with Mana:


Runes:
MPen Marks / MR marks - depending on ur playstyle as the runes suggest
Vitality seals - squishy casters who aren't planning to build RoA
Mana Regen seals - Mana heavy champs (generally mix in a bit of these with vitality seals
AP/lvl seals - when you opt for late game
AP/lvl glyphs - most times
MR glyphs - Against gay laner
Flat AP quints - most times
MR Quints - Against gay laner

Masteries

21/0/9 - 90%
21/9/0 - Against gay as hell laner

Items
Start boots 3
Get a Doran's Ring or two

If playing a relatively close range champ - get Kage's
If playing an autoattack-after-skill champ (eg: tf, fizz) - get Sheen (DON'T GET ON CHAMPS WHO DON'T NEED IT)
If playing Ryze (or possibly Kass) - get tear
If playing late game heroes (eg: Karthus) - get RoA (DONT GET IT ON CHAMPS WHO DONT NEED IT. MY LARGEST PET PEEVE)
If against high burst champs - Get abyssal
If going fine - get Rabadon's
If needing safety in melee range (eg: Morg) - get Zhonya's
If needing to kite - get Rylai's
If needing sustain - get Revolver (and possibly finish WotA)
If needing extreme mana regen and opting for late - get Chalice (situational)

Generally want void staff as soon as enemies get 2 or more Negatron's
Always aim for a big item after a small one. ALWAYS.


AP Mids with no Mana


Runes:
Same as mana cept w/o mana regen

Masteries:
Generally 21/9/0 unless you're Kennen

Items
Start boots 3
If close range - get Kage's
If need sustain - get Revolver
If against heavy burst - get Abyssal
If needing safety in melee - get Zhonya's
If already have another caster item / fed as hell - get Rabadon's


Something like that.