Do you remember drawing on a chalkboard at school? Usually, teachers use chalkboards for writing and sketching figures, but very few use them for drawing. However, a chalkboard is inexpensive and easy to use. You do not have to buy drawing paper: all that you need are chalk sticks (fine grained) and a home-made chalkboard you can erase again and again with a wet sponge.
To build a chalkboard, cut a piece of good quality plywood and paint it with a special chalkboard paint (preferably black for artistic drawing). Now you can draw without being restrained by the small proportions of a regular sheet of paper.
At some point, I attended Morphology classes at the École nationale supérieure des beaux-arts in Paris, where the human shape is taught using chalkboard drawing. The setup is simple: three-meter high chalkboards are fixed to the walls surrounding the anatomy amphitheater. In the center, standing on an estrade, one or several models are posing. You can either look at them or draw on the wall, but not both. However, you can draw them at a one-to-one scale.
Usually, you are smaller than your drawing: it is no longer your hand that moves, but your entire arm, even your body. This frees you from uneccessary detailing and let you focus on the essential shape. For those who want to try extreme viewpoints, there is a second series of chalkboards that you can reach by climbing a ladder up to a balcony.
It is amazing the number of subtle effects you can achieve with chalk sticks: from heavy, thick lines to light hatching work. Moreover, by using charcoal, you can play with light and shadow, as you would do on paper. Needless to say, a blackboard with chalk is far superior for drawing than all the whiteboards in the world, with their expensive, polluting markers.