.Here's a demo sketch I did for my head drawing class, I thought I'd pass along a few pointers to those of you interested.
First, simple but accurate construction drawing then massing the light and shadow, this should be as simple as asking yourself what's in direct light and whats in shadow. Then addressing the turning forms within the light and shadow and designing hard and soft edges.
This overlay shows how a simple construction drawing lays the foundation. Though it may seem a bit abstract, it helps place the larger masses of anatomy within the whole, it maintains the symmetry of the head and helps convey the simple three dimensional nature of the head (like longitude and latitude on a sphere).
Always look for the simple statement as shown in this photoshop paint-over. If you put anything (and I mean anything) in your drawing that varies from it's simplest shapes and values you'd better have a darn good reason. Variations in shape and value that don't convey form to the viewer (even if you believe you're really seeing them) are the first and fastest step to drawings that don't hold together. Remember that your eyes want to find as much information as possible and tend to notice variations and contrasts more than they notice the overall value and shape relationships. squinting your eyes down to cut out all the surface details to see the simple massing of shape and value isn't just a good idea, it's a necessity.
Let me repeat it because this "simple statement" paint-over describes the most important thing I know about drawing. So few people ever master drawing because they don't know or are not willing to find the simple masses and relationships underneath all the complexity they can see. Everyone notices how the parts contrast from the whole, but you have to forcibly train yourself to do the opposite, to notice how the parts most simply relate the the whole as described in this final image.