If you're going to be standing all day, stand on an anti-fatigue mat. They are used in industrial settings to reduce strain from standing, and they do make a big difference.
I use a standing desk full-time and find that two anti-fatigue mats stacked on top of each other are better than one. But one is far better than none. Haven't tried three yet. :)
Get the big and thickest one, the small is just too small if you stand with legs slightly apart.
It trains your muscles while you stand.