Chapter 3 of Vilppu's drawing manual teaches drawing the human figure as a group of boxes. I did this both slowly, and in detail during figure drawing sessions, and quickly drawing in parks and coffee shops. I found this exercise very useful because it forced me to conceptualize the subject in three dimensions in perspective. Of course, humans are not square. In order to render the figure, I had to use shapes beside boxes. However I still kept these shapes geometrical, with clearly defined edges. This really helped my understanding of using planes to describe form. I feel my drawings are more structurally sound, and better conceptualized in three dimensions as a result of the time I spent on these exercises even when I am not drawing boxes.
Critique is always appreciated.