I buy the rolls of fly tape at TC. I then put it on the wire outside the covered areas of the runs. They collect loads of flys, nats etc. This keeps the population of flys from multiplying so bad. It also lets you know if there are fleas etc. I start in early spring. I even put strips outside the reg. run where the chickens tend to congregate. I tare pieces off the place here and there on the wire of the runs. Always outside the run facing in. It stricks on its own but I use zip ties to make sure it stays put.
http://www.domyownpestcontrol.com/fly-ribbons-p-1324.html They also come in a larger size on a reel. I have been using these the last few years and really like them. My main coop is in an old bank barn and I have them up in and around the coop in other places in the barn. I try to start when I see the first fly and it has really worked to keep the population under control.
They are really really sticky the chickens will not be able to get flys off of them. You really want to keep them out of the reach of the chickens since the glue will make mess of the feathers if they really get tangled in it. But, not following my own advise, I actually will hang them to ground level under the ladder to the roosts and the chickens don't bother them. Occasionally a chicken will get chased into one or something and you have to peel it off, but the glue goes away in a few days when they dust bathe.
Vanilla scented auto air fresheners-the little Christmas trees. I get them at Wal Mart in the auto section and hang them in the coop. It keeps the flies away and smells good too. If your chickens are attacked by buffalo gnats, wait until they go to roost, then spray them with vanilla, diluted with water about 1/2.