Moving down I detailed and shaded the arms. I shaded very lightly at first, and darkened the shadows under both arms, especially the shadows under the right arm. The right hand is in more light, so I darkened the shadows on the right arm to make the contrast between the light on the hand and the light on the arm all that more pronounced.

Here's a tip: Sharpen your pencil when you're doing really tight and fine detail, such as in the face and the right hand. Then let the pencil dull itself as you shade in the larger areas. The duller your pencil, the smoother the gradient you can create.