When you sit for extended periods, your body (shoulders, neck, lower back) often suffers.
Here's a tip a traveling salesman once taught me. Every 1/2 hour or so, when you drive a long distance, change the position of your seat. Move forward a notch, back two notches, tilt the seat back, raise or lower the steering wheel--any combination of movements will work.
Not only will it help reduce muscular stress and stiffness from sitting in one place too long, but it will also help keep you alert, awake, and refreshed.
Try this same tip at work or home when you're using your computer. Set a 30-45 minute timer to remind you to lift or lower your chair, change the height of your keyboard or change the tilt of your monitor. You'll get the same benefits as in the driving example above. If nothing else, just 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 or earpiece 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!
