Flash and other reflections on eyeglasses can be distracting, especially if they obscure the subject's eyes. Here are suggestions for avoiding flash reflections on eyeglasses:
1. Use a lens with a longer focal length if possible.
2. Position the light.
a. If the flash is on the camera (e.g. popup flash), then use a three-quarters view of the subject instead of a frontal view.
b. If the flash is off-camera and the subject is at a three-quarters view, then use on-camera flash OR put the light just a bit to side of the camera, or behind and to the side of the subject.
3. Try having the subject's eyeglasses tilted downward, or have the light at a higher (or lower) angle to the subject (instead of eye level).
Unfortunately, there's no simple rule of thumb that works all the time, therefore the foregoing suggestions are admittedly kind of vague, which is why I suggest understanding the science behind the suggestions (below) in order to determine the exact suggestions that will work for your particular situation.
THE SCIENCE BEHIND THE SUGGESTIONS:
If the light is within a certain zone relative to the reflecting surface and the camera, the light will be visible as a reflection in the picture. That zone is called the "family of angles" and varies depending on the 1) width, 2) positions, and 3) angles of the A) light, B) the reflecting surface, and C) the camera.
To identify the family of angles, you need to do a diagram.
1. Draw the camera and the eyeglasses (which can be represented by a short line).
2. Draw a line from the camera to the left edge of the eyeglasses.
3. Draw the reflection from the line in step 2.
4. Draw a line from the flash to the right edge of the eyeglasses.
5. Draw the reflection from the line in step 4.
If the flash or other light source is between the lines in step 3 and step 5, then the reflection will be visible in the shot. If the flash is not in between those lines, the reflection will not be visible in the shot.
In the diagrams below, the blue bar represents the eyeglasses. The dotted blue lines are imaginary lines perpendicular to the edges of the eyeglasses. The green lines represent the lines in steps 2 and 4. The red lines represent the lines in steps 3 and 5.
In the top left diagram, a camera with on-camera flash is squarely facing the subject's eyeglasses. The on-camera flash is in between the red lines, thus the flash reflection will be visible (symbolized by the yellow color).
In the top middle diagram, the flash is off-camera. The flash is not in between the red lines, thus the flash reflection will not be visible (symbolized by the white color).
In the top right diagram, the flash is off-camera, in the same exact position, but a wide lens is used instead, creating a wider family of angles. Although the position of the eyeglasses and the flash haven't moved, the flash reflection is now between the red lines and will be visible.
In the bottom diagram, the subject is facing a three-quarters view. An on-camera flash will not have a visible reflection. An off-camera flash might not have a visible reflection if it is not too far off to the side. If the off-camera flash however, is placed far enough to the side, it will become visible. With a larger light source (such as a softbox, umbrella, or bounced flash) there is a higher chance for at least a portion of the light source to be within the family of angles.
In analyzing the family of angles, don't forget to consider that the family of angles is three dimensional. A flash reflection that might otherwise be visible may be avoided by having the eyeglasses tilt up or tilt down, or by putting the light source sufficiently above or below the plane of the eyeglasses.
WHAT ABOUT OTHER REFLECTIONS?
Although the foregoing assume a flash or other light source as the source of the reflection to be avoided, the same process can be used to avoid any other reflection with one difference - you can also cover an object as another way of avoiding reflections. As long as the cover is dark and not shiny, its reflection won't show up! :)
In the shots below, I used a Tamron 18-250 on a Pentax K100D, at 250mm (around 375mm equivalent). Because of the long focal length, there was only a narrow family of angles. Although I used a large light source (flash bounced above and to the front of the subjects), the reflection wasn't visible in the eyeglasses.
In this shot I used an on-camera flash, but the subject was looking away from the camera, therefore the flash reflection was not visible, as in the fourth example above.
Reflections (among other things) are discussed in detail in Light: Science and Magic, a lighting classic and a must-read for product photography. See also this related post about reflections in aquariums and exhibits.