You need to find out what the source is of your allergy and AVOID it. The common one's are usually dust, mold, pollen, pets and grass. Is your house clean? Is it damp (then it might be mold)? Are you ok outside the house? Does your laundry detergent have sharp perfume in it? Try eliminating things and see if it goes away. Maybe right down every time it happens and where you are/eat/smelled/touched?

It is usually not enough to take antihistamines if you are exposed to an allergy 24/7. Your body gets weak from fighting all the time - really - so you could actually get the flu or a cold easier because your body can only do some many things at the time.

I don't know what antihistamine your taking, but benedryl just puts most people to sleep and only helps so much. Zyrtec works for me cause it covers pet allergies as well. Heard good things about Claratin as well.