I've found Jack Cole's Instant Cartooning Course very useful on numerous occasions. It helped me a lot with hands (it shows you how to understand and get hands right with a pentagon-based system) and swiftly demystifies heads in profile.
Jack Cole was the creator of Plastic Man, he also produced many beautiful painted pictures for the then-newly launched Playboy. I took the scan above from "Jack Cole and Plastic Man" by Art Spiegelman and Chip Kidd, a fascinating overview of his often crazed, dynamic work.
Here is the text from the course - it's probably better just to buy the book but there are so many gems in here that it would be daft not to include it.