Reducing Neck and Shoulder Tension

I've seen many people sitting at their desks wearing a "stress backpack".
The best preventative for this is the following exercise:

Sit tall. Take the fingers of both hands and find the thick muscle where your shoulders meet the back of your neck. Squeeze as hard as you can - it's ok to feel discomfort, but not pain. If you're in pain, back off a little. Relax your elbows and breathe deeply for 1 - 2 minutes while squeezing. Then release and massage the area for a few seconds. Your shoulders will feel a little warmer because you've just relaxed the muscles and improved circulation. AND because you were deep breathing, you've brought fresh, oxygenated blood to the area.
I recommend completing this at least twice a day to relieve tension and stress.