Controlling fleas is hard, as they are extremely robust little organisms.  Advantage works as well an anything else to control the fleas on our cats.  In the warmer months, we find them scratching again about a month after we apply this.  When we only had one cat, an application would last longer.  This is probably because there are now more fleas living other places in our house; the carpets, their beds, furniture, etc..  Treating the animal is only part of the flea solution.  You must vaccuum religously and keep their beds clean.  Also try Program if you are having a real hard time controlling fleas.