agreed. Most of my problems come from large horse-flies that will follow me for a mile or two, never really being able to land on me, but always bumping into my face and neck. It's more annoying that anything else.
I've tried alot of things and have come to the conclusion that DEET containing products aren't very effective in deterring attacks by members of the order Diptera.
The best strategy is to change speed and direction radically, especially around large objects, sharp turns in the trail, or after you jump over/run under a fallen tree. Most Dipterans track by sight and are attracted to shiny, sweaty skin and hair. However, due to their compound eyes, their distance vision is not very good. If you can get 20-30 feet away from one, you become invisible to it.
Besides turning 90 degrees sharply and going from 7:00min pace to 800m pace for a short burst, changing habitats works well too. As soon as I get out of the forest, where its damp and dark, and into a field, they seem to leave me alone. l.