I used a swamp cooler last year with mixed success. The birds definitely appreciated the cooler air, but the humidity in the chicken cabin rose too high and the fly population exploded. This year I'm sticking with window A/C units. So far they do help, but it's hard for the little buggers to fight off 115*.
I've been using Fly Eliminators from Arbico Organics. They're local (based in Catalina). The Fly Eliminators work well. I still get minor population booms when conditions change quickly, but it is nothing like the major fly swarms I had before I started using the Fly Eliminators. Fly Eliminators are tiny, predatory wasps that lay eggs in fly larvae. I have 50-60 chickens. All large fowl. I get a 1/2 unit of fly eliminators every 4 weeks, starting in early March and running through October. If you order for a whole season Arbico will send you bonus units of eliminators, usually during the hottest, most humid months. It's not a cheap program but in my opinion it is totally worth it. I hate flies.
Edited to add link: http://www.arbico-organics.com/category/fly-control-program