I'm excited about this one because I find it so helpful! This video was made by Jose Gamboa, creator of the Sketch Aerobics program. Unfortunately the audio didn't work out so I'm speaking for him in the video.
There will be a series of sketch aerobic videos including a live demo whenever I take a trip down to The Savannah College of Art and Design where he is a professor and film one of the demos. In this video he goes through the first 4 exercises. These are warms ups you can do before sketching or in your free time.