What are the best shoes for standing for long periods of time?

I'm on my feet for 5-7 hours at work and my feet are killing me by the end of the shift, particularly my heels. I wear my puma running shoes because I don't really own any other sneakers or "work appropriate shoes." Are there any sort of sole insert things that I can buy for my shoes or should I just get new shoes? If so, which shoes do you suggest? Oh, they have to be closed, so no sandals or anything of the sort, and preferably sneakers.