I spend about 12 hours per day on a computer and, despite trying to stay fit, i pretty regularly got back or neck ache.
a few months ago a friend suggested trying some software called postureminder that uses a webcam to check how you're sitting and reminds you when your sitting badly. the idea is that we all know how we should be sitting, but often forget and need a gentle nudge in the right direction from time to time.
i've been using it for 6 months now and swear by it. at first i was a bit skeptical and it was a bit irritating to begin with - but that's cause my posture was so bad at first from years of computer work (i'm a programmer). now i get much fewer reminders because i've greatly improved my posture and feel better at the end of the day.
there's a free trial on their website if you want to try it for yourself, i really recommend it