John Asaro instructs to rotate the ball and draw it at all angles until you can draw it correctly from memory. You do not realize the difference it makes when you actually start drawing from memory. I've got so many questions popped up in my mind when attempted to draw it without looking at the ball. It made me think and analyze how lines change and move. I am learning tons!
P.S. September 20, 2010. I have discovered a way of drawing a ball at all possible angles without having to memorize it. You just construct it using Andrew Loomis's method. See this post