Head construction is the key to drawing any sort of likeness. I like the paint can method over the sphere method.
www.kurtaspland.com
Kurt Aspland is a graduate of the Art Center College of Design in Pasadena, California and has been drawing, painting and teaching for well over 30 years. He has been published in multiple magazines including The Disney Magazine, Forbes, The Wall Street Journal and Dallas Morning News to name a few. He has taught at Drexel University in Philadelphia and the Pennsylvania School of Art and Design.