Flee prevention

My cats have been flea free for over a year now but every time I see one scratch I worry. Barney is allergic to flea bites and the last experience was horrible. He lost all of his hair on his tummy and back. Despite the fact that I gave them Frontline or Advantage monthly and kept them indoors they still got fleas. With the help of the vet and some prescription flea treatments we got rid of them. I stopped giving them over the counter flea meds because they obviously don't work and were a waste of money. I was wondering if anyone had any other ideas of what I can do to prevent another infestation. What has worked for you? 

Being that you live in San Diego (I was born there, and my mom says when I was a baby our house had a terrible flea infestation and we didn't even have any pets!), I might suggest that you keep them on the prescription flea meds year-round, unless they had a bad reaction. What meds did the vet prescribe?

The medication the vet gave me was called Comfortis. It was originally for dogs but then made available for cats.  That combined with a flea killing spray on the carpet completely took care of the problem. The vet did not suggest I give it monthly but only when there was an infestation. Seeing as how the over the counter flea meds did not prevent the fleas last time and I know of others that have had that similar problem I feel they are a waste of money. I'm just looking to see if anyone has other ideas. Thank you for replying and your advice! 

Comfortis can be used long-term. I wouldn't want to use the spray all the time, though, because those are very powerful pesticides. But I don't see any reason not to use the Comfortis as often as necessary.