My mom always has us weave fabric softener sheets between the laces in our tennis shoes when we hike, because it is supposed to keep the ticks away, and as far as I can tell it works. Idk if it works for flies too, but that's what I use them for;)
Where I'm located in Louisiana we've historically had problems with flies of all types, from the small biting ones to deer flies and HUGE horseflies. I started searching for a natural fly repellant and came across several. The one I tried was: 1 cup vinegar, 2 cups water & 2 Tbls of Dawn dish detergent. I put it in a spray bottle and applied it every morning when I fed. It really helped.
However, I also talked to several people who put 2 oz of apple cider vinegar on their horses feed 1 time daily and use it as a feed through. I tried it and have noticed a big difference within 2 weeks. I no longer use the spray and my horses have far fewer problems w/flies.