(At the further risk of alienating everyone younger than me; I'm only barely old enough to remember records, and never actually owned any...)
Get a standing desk. No slouch. Good stuff. Treadmill gizmo is completely unnecessary.
The hardest thing I found was maintaining the right balance: avoiding locking the knees, and not shifting weight primarily to one leg or the other, which would give me hip pain.
To relieve the pain, I eventually found myself leaning more and more of my weight on the desk (which was only slightly below by elbows), and I'd have to catch myself and stand back up straight again.
Moving back to a normal desk and Aeron chair, I was in giddy heaven for the first week, it was that wonderful.
As my friend put it - "Standing desk doesn't mean you're only standing. It just gives you the option to stand or sit."
That rant over with (sorry, again), I just went to the local hardware store and got some cinder blocks. The price difference (~$7 for the cinder blocks versus ~$1000 for a height-adjustable desk frame) was worth the slight lack of convenience. That, and without the adjustability, I won't be tempted to just leave the desk in the 'down' position.
However, I did have someone with a Ph.D. in kinesiology (I think?) tell me that there's essentially no evidence of knee, hip, or back dysfunction in developing countries where people don't sit all day.
Here's a somewhat nutty article on it called "third world squat": http://www.t-nation.com/free_online_article/sports_body_trai... but it is an actual thing that people and it makes for a different muscle development than chair-dwellers have.
I think that helps me focus more, and it's probably one reason that I don't get sore from standing in one spot. Others may experience different results.
I'm a part-time stander; perhaps 25 hours per week. I also sit, recline, and every once in a while decadently lounge while reading super-important work email on my iPad. Still, I've been standing a lot for many months, so I can look back and observe:
* It takes a few weeks to feel right. I'm far past that, and I feel much less tired after standing most of the day, than sitting all day.
* Standing a lot is really not a big deal. Lots of people work on their feet all day. Every day. See the above comments about ancestors.
* It is probably possible to develop all manner of trouble by standing too still. I wouldn't know; I am essentially incapable of standing still. I shift around continuously as I work.
* It is indeed very helpful to have the monitor high enough to look straight at it; this has totally removed the incentive to slouch.