Bhujangasana or Cobra pose. I've been neglecting backbends in this series and that's gotta stop right now! Backbends are tough for me...and for a lot of others too. While some people are naturally very bendy in this manner, most of us think deep backbends are not possible. However, Cobra pose is the gateway to deeper backbends. If you can master a decent, well-aligned Cobra and, of course, practice it often, you'll be set for Urdhva Dhanurasana one day. http://www.youtube.com/watch?feature=player_detailpage&v=XU0wJ0OTopU
It's all about using the rhomboids. The muscles in blue are strongly engaging to not only life the chest & shoulders away from the floor, but propel them forward as well.