Demo of a marker sketching technique I like to use to discover new shapes and volumes. Here's the Gnomon Workshop video I mention and my Instagram ID if you want to see more of these kinds of sketches after a bit of iPhone processing.
Instagram ID - scoro5
Gnomon Workshop - http://www.thegnomonworkshop.com/store/product/331/Creating-Unique-Environments#.Umm_5BZE9As