The skill sequence isn't an exact science, you'll need to adapt to the combat, but i've found that this works in most of the cases.

AD champs are specially susceptible to the Blinding Dart, so it's the most important skill in Teemo, since it denies the physical attacks for a small ammount of time.

In most cases I always use Flash and Ignite, mostly to avoid champs running away in early combat, because in the beggining Teemo doesn't do much true damage.

Ignite is also very efficient in "****-blocking" some champs, namely Xin Zhao that has high life-steal hits. Since it reduces healing, its a great way to leave them with less life in case you die, making them an easier target for your team.

Farming is extremely important for a good build with Teemo, and with the Poison shot, you'll need to pick the exact time to get last hit kills or you'll simply waste the minions and get only XP, not that XP isn't important as well.

Mushrooms are excelent in crowd control, so in case of a large minion attack, use a mushroom trap to farm them all, securing the lane and gaining the gold.

Runes are still a bit of a new subject and the correct choice is still a mystery to me, but I believe that this line-up is, at this moment, a good choice.