Not only are we killing ourselves by sitting all day, we're probably sitting all wrong. Esther Gokhale, who has studied the posture of people in less industrialised places (where back pain is virtually unknown), shows us in this video what natural ("primal") posture looks like for standing and sitting.
Essentially, you want to have a "ducky butt, not tucky butt," she says in a profile of her work on SF Gate. Instead of tucking your tailbone, stick your butt out, because good posture relies much on the pelvis.
If you don't have time for the entire video, go to the 4:25 mark to see a sitting exercise that will help you get back into your primal posture.