*knock on wood*
I've never seen a flea on my dogs, except years and years ago. Our Golden was allergic to fleas and even a few bites sent him into a hot spot mess..
I used to use Frontline (Plus), but for over two years now I used Advantix, Advantage and Frontline (Plus) in rotation / depending on the time of the year. I've heard MANY stories about Frontline not working, it was recommended to me by several vets to rotate. I am a professional pet groomer and this is what I recommend most of the time. Also, keep your pet dewormed.
They key is to use it as directed and make sure it is coming in contact with the skin.
Make sure if you have fleas that your home AND yard is treated. Also treat your vehicles if your pet goes for rides.. Educate your neighbors too and make sure they are using a flea preventative.
For investations I recommend Capstar.
Revolution is excellent, but it's hard to find it in stock when I need it without ordering online.