"When you wish to remember well something you have studied, do this: when you have drawn the same object so many times that you have it in your memory, try to execute it without the model. Now trace the model on a thin smooth glass, and place it upon the drawing you have made without the model, and note well where the tracing does not coincide with your drawing. Where you find that you have made errors, resolve not to repeat your mistakes. Return to the model and draw the wrong part again so many times that you will have committed its image to the mind."

-- Leonardo Da Vinci