I just use my desk for different purposes. I sit while watching videos or gaming, and I stand when foruming, typing or browsing the web. I make sure to alternate. I also sit in a different chair when reading. Shifting around really helps.
So, using my "super cheap" strategies, I have a pile of old computers, and my CRT monitor and laptop are at the top for standing. Next to that I have my other CRT for the other computer. If you do have two monitors and a desk, you can get a VGA switchbox to change to the "sitting" monitor, then just sit in the chair when you want to sit down. Having a second keyboard/mouse is also needed (You can leave them plugged in, or switch with a KVM.)
I like standing, and I shift around a lot. It's also good when my room-mate bothers me with whatever: I don't have to look up at him.