Amazing sustained damage
Great poker as well
Easily kills tanks
Amazing late-game once the build is complete
I easily get at least 13 kills a game with this build
Teemo can chase down enemies and kill them quick
This build also works for mid play
Great addition to team fights

Preferably be top with a tank, as teemo is extremely weak early game. Stay near the tank and try to farm minions. Let your other player start champion fights,and always stay close to him. Be extremely wary of any champion that can pull you to them or bring you towards them in any way. As soon as you start leveling, upgrade blinding dart and toxic shot as much as you can. If you play teemo well, you will only occasionally need extra speed to escape conflicts, and anyways you have flash there to help you. Focus on not dying the first couple minutes of game play, and pick up kills wherever you can.

As soon as you get your mushrooms, you need to change your game strategy. Place them in the river so you won't get ganked easily. Also you will not need your tank as much, as you should probably be close to having ionic spark. Work on luring enemy champions into grass where you have placed mushrooms, for example running in there when they start coming after you. Try to annoy them as much as possible, it will drive them crazy and make them a lot weaker. Again, your focus is not to die. As soon as you get Ionic spark start farming like crazy, you don't even need to last-hit minions. Try to target the minions closest to the enemy champions, as the lighting magic damage will hit them as well. This is the time of the game when you should focus on farming and really start getting kills. Once you get the black cleaver and tiamat, you are pretty much home free. Your damage will be amazing, and you should start concentrating on enemy champions.

The infinity edge is really just icing on the cake. The other team should be extremely scared of you, and focus on killing the isolated champions in the lanes. This is when your team should start team-fighting, but your damage will be so amazing you can easily handle yourself. Don't be scared to go your own path and kill some champions! Place your mushrooms everywhere, and just annoy the heck out of the enemy team. Also, in close combat fights where you are evenly matched, be sure to place a mushroom in the middle of your fight right next to them, as it does a crazy 400 magic damage plus poison! Enjoy this crazy creation you have made :)

Place your mushrooms anywhere where there is grass. This will give you and your team ideal vision to counter ganks and make quick kills on isolated champions. Also, in the heat of battle place mushrooms right next to the enemy champions, as they do a lot of damage and can really help if you need to make a clean escape. Don't be afraid to help out your fellow summoners if they are fleeing the scene, place a couple mushrooms in their trail to slow their pursuers!

There are a couple of champions that you need to be wary about:

Alistar (knock back)
Singed (poison+knock back)
Katarina (burst damage)
Bliztcrank (his pull spell)
Heimerdinger (turrets)
Ashe (she can slow you down dramatically)
Any champion that has a significant shield (teemo relies on quick damage to survive)

The Black Cleaver: Great attack damage and overall a good addition to you AD teemo build.
Witt's End: Deals an extra 42 magic damage each attack as well as increasing your attack speed dramatically
Infinity Edge: Amazing damage, icing on the cake
Tiamat: Gives you extra damage, area attack good for farming creeps
Ionic Spark: Great for farming creeps, gives extra health and attack speed. I have also gotten multiple ocassions where my ionic spark gets me a multi-kill through it's chain of lightning.

Ghostblade (Great Active Spell)
Boots of mobility (Very fast if the enemy team has a lot of fast champions)
Warmogs Armor (If you are getting killed fast, this is a great counter item)
Blade of the ruined king (If the enemy isn't engaging in team fights, this item is great for killing lonely champions)