There are a whole slew of these things floating around YouTube. Picasso, in my mind, was an undisputed master, and the thing to remember here is that his working process was always different. Sometimes it was a straight shot to a finished painting, and sometimes he would just keep tweaking it until he was satisfied.

I'm paraphrasing, but my favorite quote of his goes something like, "I paint like the Chinese. I don't copy nature - I work like her."