Remember to check your posture a little and often when at your desk.  Despite best intentions, it is easy to slump into poor posture.  Often the first indication that we have been working in a poor position is pain - by then it's too late!  Set your computer or mobile phone to 'beep' or alarm in some way, reset each hour and use this to check your posture.  Are your feet flat on the floor, wrists neutral, shoulders back?  Make sure your chin is not poking forward and that your mouse hasn't migrated too far from your keyboard (causing you to reach, placing strain on your neck and shoulder).