Hydroxyzine which is a longer acting antihistamine, and seems to provide better relief than Benedryl short term.

Also supplement your dog's diet with fish oil. The Omega-3 fatty acids have a mild anti-inflamatory property. For a 70 lb dog one can give 3000-4000 mg 2X daily. You can scale the dosage depending on the weight of your dog, and its reaction to it (e.g. stools may become a little soft initially, but should revert to normal with time). Vitamin-E during the itchy times may help the skin to recover better. Not recommended to give Vitamin-E supplementation continuously.