What we did, when we had cats that would go in and out of the house, we would put Brewers Yeast on them. We put a little bit in the ruff of their neck and a teaspoon mixed into their wet food. That's the natural way of going about it, and if you wanted to get something more "medical", you could put the prevention drops by Revolution on them. They prevent your cats from getting fleas, heart worms and ear mites. Our current cats don't go outside, so we don't need it, but my Aunt uses it on her cats and she says it works great.
Also, I'd stay well away from flea collars. There are too many flea collars leaving huge chemical burns on cats and dogs lately.
