First off let me say this build focuses more on Teemo's poison/attack speed and may not work in all circumstances. However, I believe this gets a good balance of ap into his basic attack, and doesn't throw it away like most AP characters do with theirs.

These are very flexible depending on how you build your ap. I like a little bit of spellvamp in there, as you'll find your health will regen a bit better over time, especially early game. I think it is useful. I throw in a little bit of MR in the Masteries in case you're taking mid with Teemo, and some armor for those freak ganks or weirdos who play ad mid. :D

Teleport and Ignite are my favorites with Teemo. I think they are extremely useful especially once you get Teemo's ultimate. I've gotten many a kill by suprising the enemy by teleporting to a shroom and popping out of the bushes for a gank. Your ignite comes in great handy with Teemo's poison. Usually I will ignite a little early, allowing it to wear down their health if they are running, and get a few hits so my poison can finish them off. Grabbing your poison first early game allows for you to utilize your basic attack against ap mids that only have one ability and whose basic attack doesn't deal nearly enough damage to you. Be mindful of the OP minions though!

Over all this build has worked really well for me. You may consider playing around with some of the build sequence to fit your needs. If your enemy team begins to build MR, I would try replacing your Zhonia's Hourglass with a magic pen item, or your Berserker's Greaves with Sorcerer's Shoes.

Don't forget to spam your shrooms in bushes all over the map. You'll be surprised how many kills you'll pick up from champions that are running away from fights to survive. I suggest getting into the enemy jungle quickly and then out if you get a chance to sneak in and place shrooms. This will help ward for your team, and maybe pick up a kill off their jungler.