From Photography Tip Blog:

Remember to be aware of and study the main source of light. The main source of light may be a direct source of light, sunlight deflecting from a wall or sunlight diffused by clouds. If the key light is directly behind, or not more than 55 degrees to the right or left of the camera, the image and colors will be clearer. Also, do not forget that ‘head on’ lighting is flat and uninteresting. Light at 45 degrees to the subject would be more interesting in most cases.