For nearly 6 years now I have run a weekly sketchclub in a local pub. We practice our sketching skills by doing live portraits which are limited by a timer. We take turns modeling for each other for 10 minutes at the time, for 2 hours. These short sketches compare to practicing the scales for an instrument. You learn to look and observe, and train your eye-hand coordination rather then aiming to create master piece in 10 minutes.