My 8th grade art teacher did this to her class. We were each given a small sketchbook and told the following:
Every day, draw your non-dominant hand, in five different poses. Spend between 3 and 5 minutes per hand. Do this for a month.
Then, after that, she made us spend a month on feet.
It was hell while we were doing it, but it worked, and the progress was easily visible. (Especially when you're starting from nothing as a young teen.)
On an unrelated note-- I finished the page I was working on, today! Ten hours, not counting scanning and retouching in Photoshop, and done a day early. (I give myself a week deadline per page.) Not bad! I think I might be getting faster, too, because I used to take 12-14 hours (or more) per page.