Here's my photo reference, Drew her out in pencil, pen, and marker, then added highlights in Photoshop. No real reason I "tripled" her, just thought it looked cool. Using this as my current screensaver. You guys dig?
There are a few areas that definitely need some attention, the way they are now kill any illusion of volume. the lips need the shadow on the right, the teeth need a shadow from the lips and the eyes need a shadow from the eyelids. The neck should also be darker than the face. Lastly, nothing you can do now but don't forget to incorporate those strong highlights on the skin even before you put the marker to the page. Plan it out to allow yourself the appropriate blank paper to accommodate them. I'd also like to see you get away from using black lines to illustrate the strands of hair. That ought to be done entirely with the hair color and its shadows/highlights. I think the quality of your work will go through the roof if you get this technique down. green/blue/purple should be used for skin shadows with normal or flash lighting. Its their complement of course, but it also simulates the blood beneath the skin giving it a glow. Her skin shadows, from my eyes, appear greenish as the surface is a bronze