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.
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.
Guidelines for Retail Grocery Stores
Ergonomics for the Prevention of Musculoskeletal Disorders
U.S. Department of Labor
Occupational Safety and Health Administration
OSHA 3192-06N
https://www.osha.gov/ergonomics/guidelines/retailgrocery/ret...
Use footrests and anti-fatigue mats in areas where workers stand for prolonged periods. Standing on anti-fatigue mats, as compared to bare floors, provides a noticeable improvement in comfort.
There's a few designs. They usually look something like this: http://images.newpig.com/wcsstore/NewPigUSCatalogAssetStore/...
or behind a bar or in a kitchen they usually look like this: http://www.madmatterinc.com/images/comfort_flow_thumb_8vhy.j...
If you ever watch Bar Rescue with Jon Taffer he gets really upset if he goes into a bar and there isn't anti-fatigue mats behind the bar.
A lot of people put them in their kitchen for when they are on their feet preparing food or doing dishes.
http://www.amazon.com/Genuine-Joe-Anti-Fatigue-Beveled-3-Fee...
They are not well known enough. The difference you feel standing on bare floor all day vs an anti-fatigue mat all day is the difference between night and day.