Supposedly brewer's yeast mixed in with food can repel fleas. I've never tried it though. I am not a fan of overmedicating but I don't want fleas so I use
Zodiac Flea and Tick Sprays - Flea & Tick - Cat - PetSmart this spray which works really well at killing the fleas on the dog and repelling them from coming back! I mostly spray it on her feet and underside since that's where I seem to find most of the fleas, and I'm pretty sure she picks them up from walking through grass.