I found this tool about 5 years ago and then lost it during a job move. I've been looking for it ever since and finally found it through StumbleUpon today. Most will find it a little annoying and kind of stupid, but it was great when I stuck with it.
WorkRave is a little status tray application that tells you when to take a break, gives you exercises to do, and locks your computer so you do them. These aren't "Pump you up!" exercises, but instead blink every-once-in-a-while, keep-your-body-from-atrophy exercises.
You might feel like your doing less at first, but I guarantee you'll feel much better throughout the day and, in the end, be more productive. Over time, I learned to do some of these things on my own, but this is just a gentle tap on the shoulder.