Great shading here! I love the idea to use a gradient for the darker part. I would just suggest one little thing : for the airs, Try to make the lighten part just a bit more dark and add pur wite to do a hightlight (you can do this for the eyes too). It usually gives grat resluts.
Just a little question : did you make your line work by hand or by computer? If you did it with a computer, did you used the bezier curves or did you just did it with a tablet?
Not sure I understand what you meant with adding highlights on the "airs" (I'm guessing hairs? XD) It was meant to be an overall dark picture, so I didn't think highlights on the hair were necessary, but I felt that the eyes needed them to give it a little emphasis.
And yeah, linework using Illustrator's pen tool. :3