Samantha is allergic to fleas. Luckily, there don't seem to be many around. I occasionally do the house with Acclaim and put FL spot on on both P and S. In addition, I put the FL spray on the lower half of Sam's back, which is where the flea welts are worse, though she rarely has them now.
I read somewhere that it's best to treat the environment, as this is where fleas spend most of their life-cycle (as eggs and lavae etc); once they jump on the cat, the fleas are already dying. Regularly treating with FL does help to break the cycle, but it's best to treat the house thoroughly when the problem first arises.
Maybe you aren't using the FL often enough?