I have previously posted about sketching on an iPad and how you can use technology to improve your sketching. I created a Youtube movie to show how this was possible, and several people took me to task, saying that tracing is not sketching.
It turns out that the old masters used technology to improve their sketching and painting. Van Gogh used a user perspective frame on top of his paintings, and several masters used a camera lucida.
There are now several of these camera lucida tools available.