I do a lot of the same things edco does to get me moving just a bit during the day. I'm not in a cubicle so I have a bit more freedom. Here are other things I do:
I have a wireless phone so I pace around the room while talking (which can add nearly an hour of walking in my day).
I used to print things and only pick them up at the end of the day since the printer is upstairs. Now I get every little thing as soon as I hit print.
When I'm thinking, I pace instead of kicking back with my feet up (the old way!).
I work in a lab where tools get scattered around the building constantly. Every 30 minutes or so I stand up and take a few tools back to where they belong. The lab is much cleaner now too!
But most importantly, I walk to work. It's only a mile, but it's good exercise. Can you walk or bike to work? With gas prices what they are, you have more than one incentive to not drive!