I've been wanting to do this for years but back in 2004 I started studying the portraits on our currency. I think it has helped me a little. If you notice the lines in this etching it might help you with your shading. Notice how some lines curve with the contour of the face. Even if you don't do ink drawings or etchings you can improve your drawings if you study the etchings on our currency.

They are very detailed. An excellent way to see crosshatching at work. As well as seeing how shading sculpts the face. Thank you.
Paulette