The simplest advice you can give to a copy editor (or a writer, or anyone who hopes to make a living wrestling with this beast we call the "English language) is this:
Read.
So much in copy editing becomes apparent simply through close reading. Yes, you can memorize a style guide. But if you truly want to make stories the best they can be (and teach yourself when to bend the rules), I advise reading. It's that basic.
You can read anything, but it helps to read what you're interested in. It also helps to read things radically different than what you're interested in. You should strive for perspective.
We just do our jobs best when we interact with the language all the time. That means we constantly learn and discover. Such interaction, such discovery, keeps us alert and ready to wrestle.