Andrey Lyapichev ( Андрей Ляпичев ) AKA Weilard have started series of articles about Pixel Art on habrahabr.ru.
That’s about graphics in computer games, but not only. Composition, perspective, perspective simulation by color and contrast, etc.
Part one, part two …
That’s in Russian. If you cannot read Russian just look through images there to get an idea somehow.