Below is a good article about how bad sitting is for the human body. I regularly advice people I know who sit for most of their day, which let's face it is most of the population, to get up as much as possible. A few clients have recently exchanged their traditional desks for desks which can be raised and lowered for standing or sitting. It's really just a question of setting a schedule at work where you stand and work for the first part of your day, then lower your desk and sit the second half.
It's really just a matter of getting used to it. As a former sculptor we were taught to work on sculptures standing up. This gave us more perspective since it forced us ( actually we were forced ) to physically move away from our work and thus kept us from getting into any detailsprematurely.