The best fly spray I've used is Tri Tec 14, but there are several good ones out there. Our vet recommended Dura Guard. None of them last as long as they say they do though, and none of them seem to keep "all" of the flies off. Sometimes we will spray the horses every day or even twice a day if the flies are particularly heavy.
The best way to manage flies though is through a thorough fly management program... I.e., manure cleanup, fly predators, fly traps.... The most important aspect would be manure cleanup. If the flies do not have an adequate place to reproduce, this will greatly lower your fly population.
Tri Tech 14 is the only one that I have found to work at all for my horse. I believe he just sweats alot so the cheaper stuff doesn't work AT ALL. Tri Tech 14 is expensive but I think it is well worth it.
Ajegberg is defiantly right though, manure management and other practices are the best way to really manage flies.
I use Endure. Though it costs about $18.00 around here it's the best working fly spray I've used. It usually lasts me a long time and I do spray my horse before a ride and after the ride(before I turn her out). Flies are really bad in Wisconsin tight now so I try to keep her protected as much as possible.
I use Repel-X, that you buy and have to make it yourself. It lasts a lot longer and you can make it at the strength you want it. All you have to do is mix the spray and water. It has worked the best, especially here in OK in July when the flies are the worst!
So theres Repel-X, Endure, Tri-Tech 14, Ultrashield by Absorbine.
Anything else? I'll probably try and make a poll to see which one of these has the best reviews by people. I think when I had Samson I used Bronco Gold (or something along those lines?) and I know Dixie Stampede uses Pyranha which is just completely useless.