Also, when you list an item the category phrases you choose also act like tags. So if you pick "Jewelry" "Necklace" and "Polymer Clay" as your categories, all of those will act as tags. You can double check this after you finished your listing, at the bottom the tags will have "Jewelry" "Necklace" and "Polymer Clay" as the first three tags. So when you're typing in the tag phrases, don't use any of the words that were already used in the category, since they're already there it will be a waste of tagging space better used for different words to maximize your search results.
I hate when people complain their shop isn't doing well, then you look at their tags and they haven't even used all 13! Remember to fill out all of those tags even if you have to stretch your brain to think of new words, it's well worth the time!