I used to use a standing desk at a job that had them to save space. I only worked there 3 days a week, on weekends (Fri-Sat-Sun) answering phones and writing down orders. Halfway through the day (often sooner) my feet would start killing me. I would end up having to grab a chair and adjust the height so I could rest one knee on it, in order to keep one foot off the floor, alternating feet, for the rest of the day.
-app103
I've been using a standing desk for about six months, and had the same problem, until I got a little step-up stool and put that under the desk. One foot up on that every so often gives me a rest. I work at home, so it's easy for me to walk away if I get stuck with what I want to write next. Sometimes I'll bring my wireless keyboard with me, lying down on a couch, and type away without looking at the monitor. Then I go back and edit. :P