Here is my solution, which is FUN because you actually kill the buggers (and can do a tally after each run):
Take a 3x5 notecard and color it blue (attracts the flies). Pin it to an old hat. Cover the note card with Tanglefoot,a sticky insect trapping gel.
http://www.tanglefoot.com/products/ttcoating.htm
Your hat with a single application of gel will last weeks. My 1 pint container of Tanglefoot has lasted 3 years and is less than 1/2 used.
Deerflies are predators which are attracted to your moving head. Using the above homemade trap, I once caught 93 (ninety-three) of these bastards on a single 11 mile run. Usually I will snag 3 or 4 each mile.
Also, I think if you do it often enough you might even thin out the local population enough to make a difference.