Good, now I know what you're talking about... and there is a fix. Go to your eye care professional and have him apply a pair of non-slip nose pads on your glasses, which should at least minimize the sliping.

If that doesn't work, I would suggest getting a new pair of glasses. Something small, light weight material, like titanium, and high-index lenses. Possibly rimless or semi-rimless frames. All of these items will add up to lighter weight, maybe MUCH lighter glasses than what you're wearing now and eliminate the problem. Hope this helps and good luck.