*Homer Simpson sound* Mmm, tofuuuuuuu...
First of all, always squeeze the excess water out of tofu. I wrap mine in paper towels and set phone books on top of it.

Otherwise it takes forever to fry to get that chewy texture. Also, fry the tofu first before you add the stir fry sauce. (Unless you like soft tofu.)
Stir fry sauces:
Salty --
-- Equal parts regular soy sauce and tamari sauce
-- 1 TBS sesame oil
-- chopped garlic
-- 1/4 c water
Sweet --
-- 4 TBS tamari sauce
-- As much Thai red chili sauce as you prefer
-- 2 TBS turbinado or brown sugar
-- chopped garlic
-- a little bit of chopped ginger
-- lime juice
-- 3 or 4 TBS water
(good for pad tai, too)