Where I live the deerflies are thick in June/July. You can easily have a dozen swirling around your head, and they can make life outdoors pretty miserable. But deerflies have a weakness that can be exploited: they like to land just behind the top of your head. Enter a product called Tred-Not. It's a sticky, non-toxic patch that you put on your hat (I use a bill cap) in that very spot where they land.
A couple of days ago, I came back from a jog with 37 deerflies stuck to the patch. They barely bother me at all while wearing the patch - it's like a vacuum cleaner for flies.
What I was wondering is this: instead of paying good money for the patches (about a buck apiece), why not slap a loop of duct tape or packing tape on the hat? I haven't tried it yet, but will soon and will report back...