I use Frontline on my Lab, surreal. I'm fairly sure they have a version for cats too. You apply it once a month on their back (between their shoulders). Good stuff.
You need to check into protection for heart worms also if you don't already do so. I use Heartgard which is a once a month chewable.
Mike
EDIT: thanks for the link Doc. I've been thinking about trying ordering online. The Vet's office has gotten outrageous on their prices for Frontline and Heartgard.