I use fly strips by the oodles; I've also had good luck with the fly traps that come in big jars. It smells like rotten meat, so you don't want it close by; but it does work. Best approach of all; get rid of the poop and nasty hay as soon as you can. That's what the flies are there for, so if you make it go away, so will the flies.
For the last few seasons I've used fly eliminators from Arbico Organics
I've been happy with the results. They send a bag of beneficial insects that I put near any damp/fly prone areas every month and it keeps the fly count way down.
another vote for fly predators. i get mine from arbico organics also and am very pleased with the results. have used them for several years. they send a package once a month which i divide between the goat barn, the chicken house and the pig pen. the predators have worked wonders for me.
Fly predators do seem to help, but it is likely a little late in the season to get the most benefit from them. The work best when you are able to start spreading them just before the flies get crazy, and then keep spreading them on a regular schedule.
Fly traps can help, too. You need to be careful, though, about what type you use and where you put them. If it is a type that is supposed to ATTRACT flies by the hordes, then you want it AWAY from the animals. You don't want the flies to be attracted to the trap, and then have the animals right under it... Most of the bigger container/stinkier attractant traps need to be placed away from where your horses rest.
I've had good luck with strips - they are usually easy to deal with. I also like the yellow Starbarr E-Z traps that TSC sells for about $12/2-pack. They don't stink and can be rinsed/reused. They are WAY easier to hang and handle than strips.
I also really, really like Starbarr Quick Strike abatement strips. You can make them stinky if you want, but they work even without the stink. The flies LOVE them, land on them, and die within seconds. They are amazing - we find piles of dead flies under them daily. They are pricey - about $20 for a 2-pack, but they work all summer long.
I have stall fans for my horses, installed about 8' off the ground, angled down to the stall. The wind current at "leg level" is diminished enough that it doesn't affect the flies a whole bunch. A bigger fan might help (mine are box fans) but then you have to worry about kicking up dust/bedding.