Lighting for digital photography
The two basic problems with on-camera flash units are, their proximity to the lens and their point-source nature. Being so close to the lens means that the lighting is flat with no modelling. Any shadows that would help to define shape and texture are directly behind the subject and hidden. The small size of the light source means that any shadows that are visible have unflattering, high contrast, hard edges.
It is unusual for digital cameras to have flash synchronisation sockets, so you can't even attach an external flashgun.
Most good flashguns have swivelling heads that can be twisted to point at the ceiling. Bouncing flash like this gives a more natural looking illumination with soft shadows. On-camera flashes don't usually allow this kind of situation but a 3-inch square piece of white paper or silver foil can be fixed in front of the flash at 45° to reflect some light upwards towards the ceiling. The light that passes through white paper gets diffused and softened, silver foil directs more light upwards. Provided your camera is capable of compensating for the reduced intensity of the light, and you might have to go into 'manual mode', the resulting picture should be much better.
Another way is to buy a flash 'slave' unit. This is a light sensor that is either built-in to some flash guns or can be a small external device that connects to the external flashgun with a cable. Essentially, it is a 'magic eye' that detects when one flash goes off the one on the camera and fires-off an external flash, which can be more appropriately situated. The trick here is to use some paper or foil to obscure direct light from the on-camera flash onto the slave unit but not the subject.
Getting away from flashes completely, a video light gives constant light and doesn't need to synchronise with the camera at all. A small battery-powered video light with a reflector or diffuser gives much more control over the light and shadow quality. It can be placed well away from the camera on a stand or held by an assistant and eliminates all of the problems associated with on-camera flashguns.