We use fly repellent gels/creams with at least 10% Deet (Di-ethyltolumide[sp]). Applied at least 3 times daily on head, crest, underside of neck, chest, between front legs, back legs, tail and flanks! We aslo feed the max dose of garlic.
Our horse has gotten worse as he's gotten older and we now give him the injection from the vet twice a year. I can't remember the name right at this moment in time, but it does have an immediate effect.