I second TriTech. Endure is also really good.
I know Apple Cider Vinegar works on biting flies, I'm not sure about mosquitoes, as we don't have as many of those 'in town' where I board. I feed it in their pellets each day...2-3 tablespoons usually, but you can add more if you want, as it won't hurt the horse.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."