||...I often had the same problem. But now they force me to get up as often as every two hours. Usually we walk around the block once, or play frisbee in the park for a while.
Even if you don't get up at all though, you should move a little. Raise your arms above your head a few times. Try it!
Another cool trick: as your energy starts to wane towards the end of the day, put on a different pair of shoes. They will be slightly different than the ones you had on, and so will change the shape of your feet a bit. They mold to fit the shoes, you know. Anyway, it is invigorating.
At the very least, turn your head from left to right and back again, say, once per hour. I'd call that the minimum amount of movement.
If you aren't used to all this, don't just jump right into it. You could really hurt yourself, and then you'll have a bad attitude about moving in the future, and be even less likely to move than you are now. Check with a physician before beginning to move, and see how much movement is right for you.
I do, I do, I do