Believe it or not your brain has a whole lot to do with the success of your drawings; specifically, the right side of your brain.  I always incorporate a lesson on the brain in my drawing classes. After this lesson I notice that many of my students realize that "ah, ha!" moment.  There is a wonderful book, Drawing on the Right Side of the Brain,  by Betty Edwards that explains and demonstrates this theory.  In the book there are amazing before and after examples, and many exercises for you to try.