External Flash Units (Flash guns, Speedlights, and Strobes)
Bouncing the flash
With a flash unit, this is the easiest way to get soft pleasing light with the flash on the camera. Most flashes have the ability to have the head tilt upwards toward the ceiling. Use this indoors to avoid that “deer in the headlights look." This not only lights your subject but the entire room. Cameras these days have excellent TTL (through the lens metering) that can calculate the amount of flash needed fairly accurately so this can be done in any camera mode (AUTO, aperture priority, manual mode, etc…).
Diffuser or Modifier
Aside from bouncing, this is probably the second most popular way of softening flash lighting. Common diffusers include softboxes and shoot-through umbrellas. These are made of translucent white materials that diffuse light into a relatively huge area before reaching your subject. They come in a variety of sizes and allow you to expand your lighting creativity by freeing the flash from your camera. Now instead of on-axis lighting (flash on camera), you have the freedom to position your light(s) anywhere and achieve some cool effects.
Raising the ISO
Another way to make the flash not so powerful is to use a higher ISO (like 200, 400, or more ISO) so that the camera can pull in more available light on its own without the need for so much fill flash.
Slowing the Shutter Speed and Opening the Aperture
This works the same way raising the ISO does. It allows the camera’s sensor to pull in more natural light so less artificial light can be used to light your subject. Additionally, the aperture can be opened up (smaller F-stop number) to allow more light in.
Adjusting the Flash's Power
The amount of power fired from a flash is calculated by the camera's metering system (usually by TTL, through the lens metering). Most of the time it gives the right amount of flash, but if you need more or less light, use the exposure compensation to adjust the flash's output by dialing up or down the flash's exposure compensation.