If your cat is allergic to the liquids (Frontline, Advantage), you can try a Program Injection. They last for six months and don't allow the fleas to reproduce on a cat's body. There is risk of seeing one or two fleas from time to time, but they wont be there long.

Capstar is a pill that gets rid of fleas within 24 hours. You have to keep giving the pill anytime the cat is near fleas as its not a preventative like Frontline, Advantage or Program.