The Blind Contour Exercise
This is a traditional exercise that will help you bring life to your contour lines. If you do the exercise right, the drawing will look, not wrong exactly, but certainly odd. It isn't intended to create exhibitable works of art, so think of it as a practice, not a test. Just forget about what the drawing will look like when it's done, and work on maintaining your attention for the duration of the pose. The exercise can be demanding at times, but each time you do it, your line will get a little better.