Glare from eyeglasses is somewhat akin to 'red eye' when using flash. For red eye, the flash is too close to the centerline of the lens, so the flash gets 'reflected' by the subjects' retina.
Eyeglass reflections are similarly caused. The source of the reflected light is in the 'wrong' place relative to camera placement...say...directly behind you and a little above. The simple solution is to have the subject look at a slight angle to the camera, or, you and the camera move left or right a couple of steps. It's like following a pickup truck or some cars when the sun is low and behind you. The glare from their rear window hits you smack in the eyes. Moving left or right, or going up/down a hill removes the glare as you and/or the truck is at a different angle to the sun.
The biggest problem is recognizing that there will be glare from the subjects' glasses when taking the picture. As mentioned above, having someone with their glasses on tip them up a bit will handle the problem. Alternatively, a circular polarizing filter (CPL) should also correct the problem, but may introduce other effects that have to be dealt with, such as less light.