How do you make a character's eyes look like they are glowing?

It really depends on quite a few things, including the medium being used (paint, pencil, etc.) and the style you are trying to convey (realistic, impressionistic, comic, anime, etc.). Basically, one style would be for the artist to refrain from adding either shading or detail to the eye. Perhaps leaving out the iris or pupal. Or even inking "Kirby Krackle" around the eye (the art of inking little black circles around the area of the eye to denote lens burn from a bright energy glow).