Know Your Values
To create any sense of three-dimensional form you must have at least three value changes to make it happen. You need a light value, middle value, and dark value. Not necessarily in that order and it is not limited to just the three. The stronger the value shifts, the stronger the dimensional outcome will become. All of this holds true for both realistic or abstract work.
Some simple concepts to remember when drawing or painting to create more depth and movement in your artwork are...
Light values and bright colored objects move forward.
Dark values and dull colored objects move backwards.