Most in-camera flashes lack the power to light subjects more than 3 to 4 metres away, so a more powerful off-camera flash unit will be required. These automatically provide the correct amount of light according to the aperture used. Make sure to check voltage of older flash units before using them.
To make full use of available flash power, always wait for the flash-ready light before pressing the shutter.
In-camera flash invariably causes red-eye when photographing a person looking straight at the camera, because the beam of light is too close to the axis of the lens. Having the subject look away from the camera or turning up the ambient room light to narrow the eye's pupil, help in avoiding red-eye. Alternatively some cameras can detect red-eye in an image and correct it in-camera afterwards. Do not pose subjects too close to the background or in front of a window or mirror, as the flash will cause nasty reflections or heavy shadows.
Using flashlight at concerts or stage shows in large halls or venues is useless as even the more powerful off-camera flash guns have a maximum working range of about 10 to 15 meters, which is not enough to illuminate the actors or musicians on stage. Try getting closer to the stage or failing this, support your camera firmly and use a slow shutter speed.