Paleo Playbook: Posture:
While I am not totally convinced, Esther Gokhale's ideas are interesting:
"Forget the S-curve. A true spine forms more of a J, where the bottom part of the J is your butt sticking out, and your upper spine is flat up all the way through the neck. Years of sitting unnaturally and carrying backpacks are the likely culprit of our modern incorrect posture."