This year the flies have changed in our area. Last year, just a spritz with the oil based Pyrahna (sp?) would kill them so quick they'd be dead before they hit the ground. This year, the oil based doesn't work at all. The best results (combined with fly mask of course) has been a spray with vinegar, brush, followed by a spray of the water based Pyrahna and another brushing. Come on Jack Frost!!!
I've tried all the home made recipes and have never had any luck with them. Next year though I am going to try feeding the apple cider vinegar to help repel from the inside as well.
I've heard good things about oil-based, but one of mine(thin-skinned TB) CANNOT use it. He gets welts, I'm sure he's allergic. He's decked out in a fly mask and fly sheet, even in this heat, I feel sorry for him.