Stand up. Sitting all day just isn't natural, no matter what. Learn to do your desk-work from a standing position, barefoot, no shoes, and with a lot of little movements and stretching from time to time. And the workout ideas already mentioned are also great - build up your core. This is the way.
I think sitting is more natural than standing. Hunter-gatheres sit around a lot but they don’t sit on chairs. On the other hand they don’t stand on one place for too long and usually are on the move.
