Lighting for Highlights
Standard photographic lighting includes three lights: key (sometimes the camera flash), fill (additional front lighting to fill out the face) and separation or backlight. Adding another light to the side provides highlights on dark skin. This provides more contrast in the picture and brings out facial features for African-Americans.
Using Shadows
Shadows with dark skin provide less contrast, since the brightness of the skin is less. The fill light provides less range on African-American skin, so it may be tempting to reduce it significantly to provide more shadow. Beware of losing detail rather than simply adding shadow if you reduce the fill lighting.
Outdoor Lighting
Using a highlighting reflector or light source to the side outdoors is similar to indoor shooting, although ambient light may be much stronger and the clothing and background may complicate the shot. Direct sunlight may have undesired effects as it creates highlights over the entire face. Shading or turning the subject will help balance the lighting.
Combined Subjects
When shooting couples with different skin, the additional lighting on the darker-skinned person's side should not affect the standard lighting on the lighter-skinned person. The highlighting may be reduced so that it is not as pronounced alongside the lighter-skinned person.