As you learn more about photography, you may find yourself using your inbuilt flash less and less to avoid unwanted side effects such as red-eye, overexposed 'rabbit in the headlights' photographs and generally harsh and unflattering light. But if you learn a few basic tips you can harness the light to work to your advantage and improve your photos. An external flash unit and/or light modifying tools can also be a worthy investment to allow you to take your photography to a whole new level.
If you are shooting with a compact camera or the in-built flash on your DSLR, the cheapest and quickest way to improve your flash photography is by making adjustments to the flash compensation. The closer you are to your subject, typically the more you will need to dial down your flash output. On compact cameras this will usually be found in the menu options, whilst DSLRs generally have a dedicated flash button that you hold down whilst rotating the dial. Start with -0.3 or -0.7 and work from there, subtracting more flash output if necessary. Try to balance the ambient light with the flash to avoid black backgrounds. You can do this by setting a slower shutter speed (around 1/30) to enable enough time for the camera's sensor to record the ambient light. If your compact camera does not allow you to adjust the flash compensation, a similar effect can be achieved by setting your camera to 'Night Portrait', 'Party' or 'Slow Sync Flash' mode.
The effect known as red-eye is caused by light reflecting off the retina. An easy way to minimise this is by asking your subject to look slightly to the side of the camera to avoid this reflection. To completely eliminate red-eye and produce far more flattering light on your subject, it is worth investing in an off-camera flash unit. A speedlight will give you far greater control and flexibility to manipulate the light, and can be as simple or as complicated as you want. There are many ways to diffuse the light of an external flash. The most rudimentary is by using a plastic bag or even a tissue as a makeshift flash diffuser, but this is not recommended as the heat from the flash could eventually make it melt or catch on fire! There are flash diffusers available in all shapes and sizes, but even the most basic will soften the light for much more appealing people shots. Coloured gels can be added for creative effects or even just to give the light a warm golden glow.
The basic rule of light is that the larger the light source, the softer the light will be. You don't need a professional lighting kit to increase the size of your light source. You can shoot through a hand held diffuser disc, or learn how to use bounce flash. Look for a white or neutral coloured ceiling or wall, and angle the head of your speedlight toward that instead of your subject. The light will reflect off the surface creating a much more subtle result. Avoid bouncing flash off dark or coloured surfaces as the resulting light will be murky or have an unnatural colour cast. When using bounce flash do not use any diffuser attachments as you need a stronger beam of light to reach the distance as the light must travel further. By the same token, if the ceiling is quite high or the wall further away, you may need to increase your flash output to compensate.
Another thing to consider is the direction of the light. Lighting subjects from the front can look unusual because we rarely see things this way in real life. As mentioned earlier, bouncing light off a nearby ceiling or wall will make your photos look more natural. But bouncing won't always be an option, so another thing you can do is move your flash unit off the camera. Most modern speedlights can be set up to be used wirelessly, but an easier solution is to purchase an off-camera TTL flash cable. If you hold your speedlight at around a 45 degree angle from your subject whilst pressing the shutter with your free hand, you will get much more flattering side lighting. You may need to experiment with the flash position to avoid creating harsh shadows on the shadow side of the face. Alternatively, the shadows may be filled in if you are lucky enough to have a reflector and someone to hold it for you!