In reply to Bob Kemp:
For mountain biking, climbing and other active sports I use a venerable Croakies https://www.croakies.com "eyewear retainer". It's still going strong after more years than I care to remember; those guys must use good quality neoprene. I originally bought it because I liked the frog design on it (they don't seem to do that one any more, more's the pity).
It's not a lanyard as in the glasses can come off but not get lost, it actually holds the glasses in place on your head - but comfortably, it's not like a rubber band. And you can adjust how snug it is very easily. I just think it's a very simple but at the same time really cunning design - it's basically just a strip of neoprene tacked in to a tube at each end but it works really well.
Post edited at 21:23