The best way to avoid glare in glasses is to simply position your lights and your subject in such a way that they don’t reflect off the surface of the lenses in the glasses they are wearing. In the studio, this is relatively easy to achieve. Out on location, where you have no control over the ambient light and sometimes your subject, we might have to resort to cleaning it up in post.
In this video, Unmesh at PiXimperfect shows us a method we can use to restore detail hidden behind glare and reflections in glasses in Photoshop. He does stress that you do need to have some detail there to begin with that you want to try to bring out.