There's a product out noe called Advantix. It's similar to frontline and advantage, the main difference is that it either kills the mosquitos or repells them (I can't remember).

Ask a vet about it. I haven't been able to find it at pet stores or online yet, but, I'm sure it will be sold OTC like advantage and frontline are soon enough.

I'm really surprised no one's mentioned this, the company has put out a huge media advertising campaign and commercials for this product are on TV about every 5 seconds.