Wired's How-To Wiki has a nice roundup of time-tested relaxation advice, touching on the benefits of getting outside, quick meditating breaks and turning off email notifications. Among the tips anyone can use is a list of stretches that are desk do-able.
The stretches come from Shelter Online and are best used as five-minute breaks every hour. If you're the type who says, internally or out loud, "Great idea", but forgets it the next day, try implementing software meant to stave off repetitive stress injury, like Anti-RSI for Mac or Workrave for Windows and Linux systems. While stretching is good for your muscles and wrists, Wired's post is really about getting your head out of the game for a few minutes and getting your blood flowing, which is very good for avoiding the feel of working as an indentured wheel-turner, Conan the Barbarian-style.