I've never cashiered, but I'm well versed in standing for 10 hours a day, so I can say it gets easier on your legs and feet the more you do it.
Also invest in good shoes. And I don't mean dress shoes.
Also I find the best way to take care of the legs and feet is find the right combination of standing and walking, as doing both the right amount is nowhere near as bad as doing one of them too much, even in the same time frame. Generally you can't decide to stand around if you need to be walking, but if you're standing around and your legs hurt, start walking around the area you need to be in. Just pace back and forth if you have to.
Finally, if your legs get sore, do leg stretches when you get a moment.