Since I tend to create a lot of detailed portraits,
sometimes I need to play.
When I was in college, my favorite warm-up exercises were the blind contour drawings. Now, many (many) years later, I sometimes head back to my roots and create blind contours, continuous line drawings, non-dominant hand drawings, and various combinations of them all. When I need to loosen up, I find the most important thing is just to keep my hand moving quickly.
What is your favorite way to warm-up?