I've used Advantage for a number of years for both cats and dogs, and find it works better than anything else for flea control.  If you are a first time user, be aware that it may take a while to interrupt the multiple life cycles of the fleas that are around your yard, house and animal/s.  [And if you have a severe infestation, you may need to do additional treating of your house--I am a big fan of using a sprinkling of diatomaceous earth, which seems to be a big secret despite being inexpensive and nontoxic.]  The biggest downside of Advantage is the price, and Amazon seems to offer the best price around (especially if they have a sale).  The second biggest downside is that my cats uniformly do not like it when it is applied, but seem to quickly get over it.  I have found that with both cats and dogs it is possible to sometimes go up to several months between applications, which is good as I prefer to use any toxic chemical on them as little as possible, so you may want to try going more than a month between applications and see what happens.  I've also had good luck with purchasing the large cat (over 9 lbs) product and splitting it between two small cats.