I use Frontline and it works, but it does not last the whole month. 2 different vets recommended that I use Frontline every 2 weeks instead of once a month. That is what I've been doing for the past couple of months now and it has definately done the trick. No more itching and scratching and no more fleas.
I agree with Lacy's Mom that you need to treat your house and your yard. Then try using the Frontline twice a month. I'm sure that will get rid of the problem.