Oh, another idea is while you're drawing pick up your drawing, and turn the paper over so that the back side of the paper is facing you, then hold it against the light or in front of the mirror.
You catch all of your proportion mistakes like that. This helps a lot while I draw. It's just sometimes you get so used to your drawing that you dont see anything wrong with it. Seeing the mirror image of your drawing is like seing it for the first time alowing you to be your own critic.
Just my
MALIX