Our model today was David Abercrombie. I didn't do a demonstration today but we did take a few minutes at the end of class to take a look at our drawings.
One thing that came up a couple times today was the importance of giving visual cues of how the simple geometric volumes of the body overlap each other at joints. An example I gave was how the cylinder of the neck connects to the torso. Looking at the model from the front the neck will tend to come in front of the top contour line of the shoulders. Giving small indications of this overlap can help make the neck feel more three dimensional and connected to the torso. We will commonly just run an unbroken contour line around the whole figure, making it appear to be a single flat shape.
Next week I will do a demonstration discussing the importance of value contrast in creating the illusion of 3D form.