When you have drawn the same thing so many times that it seems that you know it by heart try to do it without the model; but having a tracing made of the model upon a thin piece of smooth glass and lay this upon the drawing you have made without the model.... where you find that you have erred bear it in mind in order not to make the mistake again.... if you cannot procure smooth glass to make a tracing... take a piece of very fine parchment well oiled and then dried, and when you have used it for for one drawing you can wipe this out with a sponge and do a second.

The Notebooks of Leonardo Da Vinci (MacCurdy, 1938) - XXIX Precepts of the Painter