Well the only way to help with flea allergy dermatitis is to get to the root of the problem. Your dog needs to be on a quality flea control. The OTC products like Hartz and Seargent are not good products and are a waste of money. Get a good product from your vet, like Advantix or Vectra.
If the allery is sever, your dog may also need oral antibiotics for any secondary infections the biting and itching can cause.