Okay... bear with me on this one. I swear it's true. For deer flies and the like, try this. You know how they circle around your head, right? Elbows at your sides, hands outstretched to left and right, palms up. Slowly at first, raise your hands and speed up the movement until you clap your hands at the fastest pace possible directly over your head. You may not get the acceleration or speed just right the first time, but, keep trying. I swear, you can kill them this way.

Of course, when there are several hundred circling you, this particular method has it's drawbacks.