I had this problem bad and although they ain't compleatly gone its hardly noticable unless you realy look.
I used neem oil, those yellow sticky strips and tryed to stop watering quite as much.
For quite a while i didnt have the stick strips and as quick as i killed the things in the soil the little flyes would repopulate it all over again.
I am interested in diatomacious earth 2 i think i will have to look it up.