If the person you're photographing wears eyeglasses, you've got a problem: How do you keep reflections out of them?
The most common reflection is from an on-camera flash or studio light. In the studio, where umbrellas and soft boxes can create huge surfaces that reflect off surfaces like crazy, the solution is simple: Have the sitter tilt her head down or away slightly so the glasses reflect the light source away from your lens. But even if the sitter shifts her head, be sure your subject is looking at the camera. If you're using continuous lights or strobes with modeling lights, you'll see the results in the viewfinder or LCD monitor.
If the light source is an on-camera flash (Move flash off the camera over to the side if you can!), the reflection will be smaller and brighter. Again, have your subject tilt her head down slightly to deflect reflections away from your camera. You can use image preview to check for reflections in your camera's LCD screen.
In this example, I photographed my wife, Lori, using two Flashpoint II FP320M 150 Watt Monolights and a pair of JTL 33-inch shoot-through umbrellas with a Canon DSLR from the Canon Store at Adorama. The umbrellas, while evening out the light, also created a large bright area that were easily reflected in Lori's glasses. By checking the photo in the viewfinder I saw the problem and directed her to face slightly downward. Problem solved!
See the subtle reflection in Lori's left eyeglass? It's gone with a slight shift of the head. Photos: Mason Resnick