I've found that one of the main ingredients of a realistic painting is a consistent, well-defined source of light. Your shadows and highlights have to be 'true' or your entire painting will not be believable. Of course, this is nothing new; all the masters had this down pat. But I've found that I can use the natural assymetry due to my 'handedness' to great advantage. I'm a very dominantly right-handed, and I find it much easier to show my scenes with the sun (or other depicted light) over my right shoulder.
My tendency is to brush from left to right, so dark colors tend to fade near the end of the brushstroke, giving a very consistent, believable upper-right illumination source. It's a real fight to try to depict a lower-left light source, for example, so I don't even try. All my paintings have an upper right light source!
Tip from: Eric Nichols
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