Posted: 18/11/2013 at 17:22
Check out this thread about a similar problem:
Chances are it is probably weak back muscles made worse by poor posture, try either spending some time on a rowing machine or doing some low weight high repetition weights. Good upper back exercises include bent over/ seated row, reverse flys, lat pull downs, pull ups etc.
Strengthening your upper back will help pull your shoulders back and head up thus giving you better posture and hopefully eliminating your back ache. Remember to work core and lower back too if you really want to improve your over all posture (recommended if you spend all day at a computer!).