Pupils turned red so often ruin otherwise great photos. And though many photo editing programs offer a digital fix for the "red-eye effect," correction after the fact never looks quite right. Fortunately, there are ways to keep pupils looking black from the start.

First, here's why the red-eye effect happens: When you use a bright camera flash to illuminate your subject's face, the light shoots through the pupils and reflects off the rear interior of the eyeball.
A layer of tissue in the back of the eye, called the choroid, is full of blood. Because blood absorbs light in the blue and green parts of the electromagnetic spectrum but strongly reflects red, only red light bounces back through the pupil to the camera lens. This causes the red-eye effect.
The darker the surroundings, the more dilated the pupils of a person's eyes are, and thus the more light from the flash they'll take in (and the larger their red pupils will appear in the photo). Consequently, the red-eye effect worsens when flash is used to brighten especially dark scenes.

The easiest fix is to turn on ambient lights instead of using your camera flash.
If fluorescent lighting isn't the look you're going for, there are other options. Some cameras come with a red-eye reduction mode that causes the flash to fire several times before the shutter opens when you snap a photo. The preliminary flashes make your subject's pupils shrink, which limits the amount of red light that can reflect back when the "real" flash hits.
Another way of reducing red-eye is to use an external flash rather than one that's built into your camera. By aiming the flash at a wall or the ceiling, you can bounce light toward your subject from the side, rather than head-on. This way, light won't reflect straight back from the eyes of your subject to your camera lens.