Because I have a desk job and can easily become absorbed by my work, I don't remember to stand up and move around unless something prompts me. There are various places you can go on the web to get little alarm clock programs that will remind you at specified intervals, but my workplace doesn't trust me to run programs they don't supply me with.
I was stumped until I realized I can do this with Outlook. I set an appointment called "standup" one day for 5pm. I told it to remind me 8 hours in advance. When the reminder popped onto my screen, I stood up, stretched, then sat down and told it to snooze for 30 minutes.
Every 30 minutes, I tell it to snooze for another 30.
In other news, it looks like winter is coming.