At the beginning of the summer I was turned on to a book of drawing exercises that just came out, and purchased it immediately. It's called Drawing Lab for Mixed Media Artists. Written by Carla Sonheim. 52 exercises that are applicable to all ages. I am really looking forward to doing a bunch of these as warm ups or full scale lessons with our Zamorano students and the class at MiraCosta.
I highly recommend this book to teachers, students, and artists alike.