Prevent the Dreaded Red-Eye
You'll usually see the red-eye effect in low light, when your subject's eyes naturally dilate to let in as much light as possible. When you fire your camera flash, the light passes through the open pupils and bounces off the back of the eye, which then looks red. That's why you'll never see red-eye in a photo taken outdoors in bright sunlight. To minimize the possibility of red-eye, take your pictures outdoors in daylight, or inside near a window where you have natural lighting. At night, brighten the room by turning on all the lights you can.
If you're stuck in a dimly lit room or if you're outdoors at night, turn to your camera for help. Your camera's red-eye reduction mode (usually identified by an eye-shaped icon) fires the flash several times quickly right before the camera takes the picture, forcing your subject’s pupils to close down to a smaller size. Remember that the picture hasn't been captured at the first sign of flash, so hold the camera steady--and warn your subject to hold still for a few seconds, to be sure that the camera is done taking the photo.
Your camera's red-eye mode can help, but it isn't a cure; you still might end up with red-eye in some photos. When that happens, use the red-eye tool in your favorite photo editor to blot out the red.