I am currently taking classes in Classical Drawing methods with Tricia Ratliff. We went through a wonderful introduction to drawing using charcoal and grey paper in which I learned more about how to draw than I did in all of my drawing classes in College.
Now we are beginning work on copying the Barge Drawings. Copying the drawings does several things, It improves observational skills, helps you learn to accurately depict form and also familiarizes the student with anatomy and classical lines. All of these things should be very helpful when working on painting portraits. I can already tell that there is improvement with my painting.
The Drawings take many hours each of very careful rendering. I have probably spent 20 or more hours on just the drawings I have done so far. Working those hours into my schedule of painting, childcare, home remodeling and occasional housework ;) is difficult but I believe will be well work the time and effort.