You've got a good understanding of shading, but you should use the burn and dodge tools less. Control what colors you use for your shadows and highlights. Say you have a yellow ball, if you used the burn tool to create the shadow it will shift the color closer to black, black shadows will flatten form. Instead try delving into other colors like red, green, or blue, any of which could be used to make a darker area of yellow. Doing this will let you be more deliberate with the feeling that you are trying to create with your image based on what colors you choose.
Below is an example of what I'm talking about. All of these balls (heh) are the same base yellow, but I've shaded them with different colors. The middle one just staying within yellow tones, which isn't wrong per-se, it's just important to experiment with color in my opinion because of how closely it ties to emotion and how the viewer will react when seeing it.
Keep it up man, your digital paintings definitely have a very dramatic effect, and you've captured the softness of the skin very nicely.