My art teacher in high school gave me some great advice. Although i thought he was a dick at the time. He told me,"no cartoons in my class. Stop drawing what you think you see and start drawing whats actually there." He also taught me the importance of looking at the human body (bones and muscles) before jumping in on an entire portrait. Start with the shape of the head and spine and work from there. Angle of the shoulders and knowing where the eyes are looking.