Ok - this is somewhat simplistic but has been a problem for me. I wear straight armed shades (as opposed to arms with a bend at the back). I also (for some of my pairs) wear them on the outside of my helmet straps. The straps cause the arms to be pushed into my skin which is very uncomfortable. This also makes it easier to remove the glasses while riding to clean them, wipe sweat away, etc.

Hope you find a cure because I get something bouncing off my glasses that presumably would have hit me in the eye (or at best eyelid) at least once a ride.