Choose locations with lighting in mind. Supplement your video shoot with a lighting kit when necessary. At the very least, light the face of your subject or the focus of the scene. This is especially important in low ambient lighting.
Keep sources of light just out of the frame. Make sure the greatest intensity of light is in front of the subject. When outdoors, try to keep lighting sources, including the sun, behind the camera.
Don't have a lighting kit?
Use available lighting. If you are recording inside during daytime, move the subject closer to a window. At other times, use available light fixtures. Tilt lamp shades or remove the shade entirely to get more light on the subject. In general, incandescent lights are less harsh and produce better color on facial features than fluorescent lights. You can also try using a bounce to reflect light and fill in shadows.