How to playTeemo is a perfect gank char, so why don't play him as DD?
Place yourself in right gank positions grass or roads (u'll be invisible after 2 seconds) und wait for your Team. With a good team u'll have a good success rate to get many kills and overfeed yourself.
If u play against other DD's (Master Yi, Tryndamere), so don't shoot ur Q skill as Alphastrike, wait for the enemy reaches u and then use Flash arrow because they won't hit u then a longer time.
Use W skill to escape in difficult situations and use ur mushrooms for Map overview. (: