It seems so obvious now but it wasn't a month ago. I was making two copies of each drawing so that if I messed up on one, I had back-up.
However, the smarter approach is to pick the part of the drawing that you are most likely going to mess up and start there. When you mess it up, you chuck it right away. Aaah... lightbulb. Or as Sumi-san used to say, "All learning is self learning."
This post is dedicated to the Hogarth sisters.