This is my build on Teemo, who is my favourite champion :)

It's based in a Hybrid Build, in order to maximize the damage in all ways :D

Enjoy!

This runes are based in a good early game, in what concerns play defensive but dealing a good amount of damage.

9x Greater Mark of Attack Speed, gives you a good begining because it helps you on Toxic Shot
9x Greater Seal of Replenishment, gives you a good mana regeneration.
9x Greater Glyph of Ability Power, extra AP
3x Greater Quintessence of Ability Power, extra AP

For masteries I use the 9/0/21 tree.

With Teemo you have to play passive because he's a very fragile champion, so the points in Utility will help you improve your mana starving and the amount of exerience you receive.

Then the points on Alacrity will give you a little boost in atack speed and the magic penetration will improve the poison's damage.

My item build, as I wrote in Introduction, is based in a Hybrid Teemo.

My first item is Doran's Ring. 15 Ability Power, 100 HP and Mana Regen. will maintain you relatively good in lane. Then I make Boots.

I DO NOT get Berserker's Greaves early. Why? It only gives you atack speed and movement speed, so it's better for you if you buy an Amplifying Tome or an Hextech Revolver, if you had enough gold. The AP and the Spell Vamp will be more viable then only Atack Speed.

Since you have some more AP and Spell Vamp, you can get Berserker's Greaves and if you have some gold to spend, try to get Malady or Hextech Gunblade for more Atack Speed and AP with the first, and Atack Damage and AP for the second. Of course that depends on the situation in game.

Now it's time to get some health: Phage is the most valiable. It gives you HP and slow, essential to get those lucky guys who are running ;) If you have enough gold, upgrade it to Frozen Mallet.

The final item is Nashor's Tooth: Atack Speed + AP + Mana Regen + Cooldown Reduction ^^ Perfect ending!

Since you have this build, you can replace Doran's Ring for Rabadon's Deathcap :)

There's another alternatives, it ALWAYS depends on the game tide ^^

For Summoner Spells I use Flash and Ignite

Flash: Very good to get someone who's running away or to escape if you are being threatened
Ignite: This is your best weapon! Ignite + Normal Atack with Toxic Shot + Blinding Dart = DEAD

Alternatives:

Exhaust: same features as Flash, but specially good reducing Magic Resist and Armor, essential to strike your enemies ;)

Well, I finished my guide :)
I'll update if you give any suggestions to the item buid, runes or masteries.

I'll wait your feedback :D