This book is proving helpful. It clarified some techniques for practicing contour line drawing.

First you must convince yourself that the pencil point is touching the model instead of the paper.

Place the point of your pencil on the paper. Imagine that your pencil point is touching the model instead of the paper. Without taking your eyes off the model, wait until you are convinced that the pencil is touching that point on the model upon which your eyes are fastened.

The Natural Way to Draw, Nicolaides, Kimon, pg 9

I always wondered, what do you do when the contour leaves the edge of the object and turns inward?

Often you will find that the contour you are drawing will leave the edge of the figure and turn inside, coming eventually to an apparent end. When this happens, glance down at the paper in order to locate a new starting point. This new starting point should pick up at that point on the edge where the contour turned inward.

The Natural Way to Draw, Nicolaides, Kimon, pg.10

And contours can lie inside the figure as well:

Not all contours lie along the outer edge of the figure. For example, if you have a front view of the face, you will see definite contours along the nose and the mouth which have no apparent connection with the contours at the edge. As far as the time for your study permits, draw these ‘inside contours’ exactly as you draw the outside ones. Draw anything that your pencil can rest on and be guided along. DEVELOP THE ABSOLUTE CONVICTION THAT YOU ARE TOUCHING THE MODEL.

The Natural Way to Draw, Nicolaides, Kimon, pg 10, 11

Helpful reminders. Now, back to drawing.