Well. I've yet to do that myself, but clearly it's an important decision made much, much harder by salespeople.
Alright. First? Get a telepath if you aren't one. If you can't do that, get a con artist. This will ensure you don't get ripped off, and will lower costs significantly.
You should know before you shop if your lady/fellow prefers gold or silver. Also their ring size. Then all you have to do is look for something that seems to suit their personality.
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