Especially if you're sitting at a computer all day. Gotta strengthen that back to keep your shoulders from curling in too much. Then everything starts to go wrong =/
The other thing I would add is core work, and not just strengthening, but also correcting posture (pelvic tilt).
All of the above just scratches the surface on how to correct the bad things that happen to your posture when sitting at a desk for hours each day.
All of this only gets worse if your typing at a keyboard because of the 'natural' position you assume.