When you sit for extended periods of time, your structure (shoulders, neck, low back) often suffer.
Here's a tip a salesman once taught me. When you drive a long distance, every 1/2 hour or so, change the position of your seat. Move forward a notch, back two notches, tilt the seat back, raise or lower the steering wheel.
Not only will it help reduce muscular stress from sitting in one place too long, it will also help keep you alert, awake and refreshed.
Try this same tip at work. Lift or lower your chair, change the height of your keyboard or change the tilt of your monitor. If nothing else, taking a regular break will help you to be more creative and to focus better.
Are you on the phone all the time? Be sure to get a headset so you're not constantly scrunching your neck and shoulder together. You'll also be amazed at the freedom you'll feel from having both hands available while you talk!