Several years ago I took an evening art course at Sheridan college in Oakville. At the start of each class we would begin with very fast sketches of a model posing in different positions then switching to a new pose.
I recently came to the realization that doing that again would be a great warm-up for drawing and painting, so I went online and downloaded a bunch of pictures of different positions, then randomly put them on screen. I give myself five minutes to do a sketch and then move on to the next one.
Over time there should be a noticeable improvement in my sketching abilities. Here is an example of a five minute sketch.