Natural or Available Light
The warm, soft light which pours through a windowsill or through the branches of a tree is an ideal way to light a portrait. Although natural light can't be manipulated as easily as artificial light, careful attention to composition will provide excellent results. Available light, especially when filtered through a thin curtain or screen, adds a complementary glow to any skin tone. Because there is little reflection with this type of lighting the subject's eyes are free to gaze, with no squinting or blinking.
Bounce Flash
Shooting indoors with flash results in harsh shadows and unwanted reflection off mirrors and other glass surfaces. The solution is to use an external flash unit with an adjustable head. This allows you to direct the flash upward so that the emitted light bounces from the ceiling or neighboring wall -- and lights your subject from above. Bounce gives the impression of a soft overhead light source even when shooting in a pitch-dark room. When using this technique, open your aperture -- size of lens opening -- by one or two stops for accurate exposure.
Diffusion
The light from studio lamps is often softened by diffusing, or scattering, it before it reaches the subject. Diffusion is usually performed by placing an open umbrella behind the lamp's head or above your subject. Often used in combination with reflector panels, this method helps conceal facial blemishes and add an attractive glow to the subject's skin.
Butterfly Lighting
Made popular by actress Marlene Dietrich, butterfly lighting masks the skin's imperfections while adding shape and dimension to the face. This type of lighting is achieved by placing a lamp several feet above and directly in front of the subject. These two light sources merge to create a butterfly-shaped shadow just beneath the nose -- as well as adding subtle shadow to the eye sockets and sides of the face. A low backlight can be placed to highlight the subject's hair.