When I first started using Evernote, I used it the way I’ve always used physical notebooks: a note goes in a notebook.
So I created a bunch of notebooks. One notebook for a school class. One notebook for my parking tickets. One notebook for reminiscing about coffee. Unfortunately, this is a fine way to miss out on perhaps the most powerful way to use Evernote: the tagging system. I discovered this system through a wonderful Michael Hyatt post. He noticed that tags are essentially the same thing as notebooks, except with a lot more power (and a lot less visual reinforcement).
Related to my Productivity Late 2014 post, the secret to using Evernote successfully is to realize the notebook — note relationship is one-to-one, but the tag — note relationship is many-to-one. Notes can be trapped and lost in a notebook, but be more fluid and accessible with the use of tags.