Also, I've heard to make sure you have anti fatigue mats for your feet. Mine were 6 bucks at Harbor Freight (only using 2 of 4).
I started last October just trying it out, and could only stand for and hour or two at a time. I just moved my standing desk to the floor when I needed to sit at the desk (laptop, keyboard, 2nd monitors and trackpad . . . took all of a minute to switch!! ). Around Dec or Jan I made the change to stand to all the time and make it through my day (~9 hrs) handily.
I'm glad I didn't purchase an expensive adjustable desk because it would just live in the standing position all the time now. If you can use a ruler and a power drill, just make your own with that IKEA trick, and switch it off your desk as needed. Doesn't take more than a minute even with wires in the back and will save you a boatload of money.
Don't be fooled though, I still have a chair. I look for chances to sit during the day (brainstorming with colleague, meetings, etc) and it DOES feel nice to sit down. But for the day-to-day, standing has been great and I don't have plans to switch back any time soon.