This site was recommended to me by a parent who found her high school son plagiarizing and found this helpful resource to share & discuss with him. Don't be put off by the name of the site or the services offered on the homepage; it seems an unlikely place for good advice on plagiarism, but the page I'm linking you to seems quite instructive and has some good links.
Covers topics like Academic Writing Services; Cheating Goes High-Tech: Online Term Paper Mills; The Web Versus the Honor Code; Paper Mills; How to Avoid Plagiarism and Cheating; Understanding Plagiarism, What it is and how to avoid it; You Wrote It, You Quote It (Interactive tutorial). It also provides links to discussions about Potential Consequences; Facing Penalties; Legal issues of cheating and what happens when students fight colleges.
The page also links to Resources for Teachers and Professors: Cheating Prevention; Anti-Plagiarism Strategies; Glatt Plagiarism Services (software that creates a test for suspected cheaters).