Insane, my mum is a mad golfer who needs glasses, and shes found that most glasses fall off pretty easily when you spinning your head around quickly so this should relate to mountain biking somewhat, Bolle Action Sport interchangeable lens with clip in perscription "plastics" are the best she's found, they come with 3 sets of lenses (clear, tinted, pink) and are about $160 without the script insterts this is compared to briko stingers and nike interchangables.
hope this helps