Start blue gold gen item and potions, yellow trinket. Leave fountain with one or two stacks of your passive depending on if you're helping the jungle or not. Try and poke with AA-Q-AA whenever possible. Try not to heal too much unless you are getting heavily poked.
Get Sightstone, then figure out who on the enemy team is going to be the biggest problem and how far ahead you are. If your team has a huge lead, you can go ahead and rush Sheen to stay ahead, but I recommend building Aether Whisp unless you are certain you are going to stay ahead. This item is cheaper, builds into other support items that you can take advantage of (Mainly Ardent Censer) if you need to build more supporty. You can also get boots earlier than if you rushed Sheen. If you go Lichbane, pick up all the AP support items, not the mid AP items, you'll get more out of them and provide a good amount of utility for your team while building damage.
I personally like to pick up Fiendish Codex as it builds into Banner of Command which has a good pushing active, as well as the MR aura, CDR, Health, and AP. From there, you can get Chalice if you really want the mana regen and then get Mikaels for the CC/heal active. Ardent Censer is good too for taking towers with a team better, and it lets your W act like a mini Q with the passive (Attack speed and magic on hit to anyone hit by a shield or heal). Ardent Censer can be delayed for Zekes. Final items can be anything you need to survive, or anything your team needs to continue being awesome.
My builds usually go:
->
->
->
->
->
or
->
->
-> 
For Lichbane, I delay banner, but generally still pick up every support AP item.
When I'm very behind, I'll go:
and boots of the appropriate defense built early.