How do you properly store an extra denture you don't need currently?

You can buy a carrying case that snaps shut. Clean them thoroughly, put them in the case, label the name, and put them in the freezer for storage.

Interesting question! There are two opposing schools of thought, one says to thoroughly clean and dry them then store in an empty denture container. They should be soaked 24 hours before being reused.
The second group advocates cleaning and then storing in a sterile liquid, I'm not sure how you would be able to guarantee the long term sterility of the of the container and liquid (presumably water?) though.
I rather liked the suggestion of one jokester to store them in a cookie jar on the counter for the grandkids to find LOL.