How many times a year can students (sometimes even the same ones repeatedly) deliver with wide, teary, and arguably quite innocent eyes the explanation, "I didn't KNOW it was plagiarism!"
But... how often does a student ask to make sure they've done it right?
I've tried the up-front detailed explanations, the slicing and dicing of what is and isn't plagiarism, and there's definitely a place for that, but now my front line of defense is one of simplicity.
This straightforward flowchart addresses potential plagiarism in both creative and more formal writing. Once it's established that the writing contains words, ideas, information, or organization from another source, every path goes through one of two recommendations: clearly cite outside sources or discuss it with your teacher.
If you love this approach as much as I do, you may also want to consider the poster so you can easily refer to it with each new writing assignment year-round: