I work at a vet's office, and we just recently had an in-service about why flea treatments seem to stop working. They pets don't really get immune to topical flea treatments, the problem lies with proper application and bathing. If you use a shampoo that contains soap on your pet, that will wash off the flea product. Also, the old thinking was that you should wait 48 hours after bathing before application. Well, that just gives the fleas time to bite and reproduce. To do it correctly, bathe the pet in a soap-free shampoo, then apply flea treatment DIRECTLY ON THE SKIN, NOT JUST THE HAIR as soon as the pet is dry.
There is a new once-a-month flea pill out called Comfortis, but it's pretty expensive. However, if you can afford it (about $80 for a six month supply) it is very effective. But you can pay the same amount for Sentinel and you get the heartworm prevention along with the flea treatment.
And yes, if your pet has a flea allergy, treating the environment is essential!!