Well, you probably don't really have to worry about ticks, just fleas, and you can probably wait until they happen before you start treating.
What has been working best for me lately is the Target version of Frontline, I think they call it "Pet Armor" or something like that. It's cheaper than name-brand Frontline, but I've also found it more effective. Go figure.
I haven't had to use it much lately. I put some on once last summer, think I saw a flea. They got a little itchy early this summer, and I put some on, but I'm not sure there actually were any fleas. Anyway, I don't expect to need it again until next summer, maybe.