In this drawing I attempted to keep the line as simple and elegant as possible, just varying the lineweight a little. Only the shadows under the hair are rendered, and the face lightly modeled. The modeling of the face has gone awry - when I drew this, I hadn't learned any strategies for tackling the head - but it is otherwise successful, I think - though I'd use the lineweight a little differently now, too.
To make tonal values truly effective, you need to move beyond simple shading and really observe how light and shade follows the planes of the figure. Take a look at this excellent tutorial by Ed Hall on Seeing and Rendering Planes in the Figure.