Well, if you haven't seen any then now is the best time to prevent them! Its SUPER hard trying to eliminate an infestation.
We alternate between Frontline and Revolution. I hate putting chemicals on them, but its way better than fleas, I think. I also do other types of preventative measures. Vacuum like CRAZY. It is the single-most effective anti-flea measure. Also check out Diatomaceous Earth. Its kind of a pain in the butt (no pun intended!) to use but really effective. I'm sure there is a thread about it around here somewhere.
I also groom Cabeza and Mabel every day, spray lavendar/sage/patchouli oil on them, and bathe them in peppermint soap.