A friend of mine had a cool product last time we went riding, a fly repellent for their faces that was like a roll-on or stick deoderant. Seemed like a good idea, for getting around their eyes without getting it ~in~ their eyes. But I haven't seen it at the store yet, and not sure where she found it.
That roll on stuff hasn't been very effective around here.
I have found the best stuff around here is Pyrhana. I only spray them down when I'm riding, and only the spots they cant' get with their tails. Petroleum based sprays will scald the horses skin if it's put on too often. I had that happen with one. Having a shed they can get in is the best thing for when they are out loose. Only little house flies will go in a dark shed/barn. Keep the stalls mucked and that will thin them out.
Becky