I had a nice discussion with an artist last week about painting portrait head studies. She felt she was pretty accurate with still life paintings, but when it came down to painting heads in a tight time frame (two or three hours is a tight time frame) she didn't do as well.
My off the cuff advice was to continue doing the still life studies, but what I should have advised her to do was to get one of these planes of the head casts. Many years ago I studied with a woman who made us do a charcoal drawing of this cast before every portrait painting. That practice really helped me understand the shapes and relative values of what is going on with a head as the different planes turn away from the light source. Naturally, this head is a prototype, not an actual individualized head and so all results will vary. But it will help you form a set of presumptions in your brain - not assumptions, presumptions -to look for when painting from life. (In other words, you can presume there is an indentation below the lower lip - but it's not always there in every subject. And if it isn't there, you might decide that this characteristic is something that you might want to pay special attention to.) It helps you to see the sculptural forms as you paint and (important when there is a timer ticking) with practice can help you make judgements a little more quickly.
This is a cast designed by John Asaro many years ago. My own cast is an plaster one (notice the cracked base, glued together when it fell out of my car. Rats.) The newer ones are plastic (gray plastic, I believe) and are more durable; they are available here: http://www.planesofthehead.com/index.php I've also seen a variety of good planes casts on the internet.