Lifehack.org has a great article on how not to plagiarize.
Some highlights:

• Don’t paste formatted text into your papers.
• Don’t use writing that is much better than your own.
• Don’t copy entries from Wikipedia.
• Don’t hand in a bunch of really well-written stuff that has nothing to do with the course or the assignment

It might be worth-while to discuss with your students why plagiarism is an awful idea, and then to actually discuss some of the common ways people plagiarize, and why it’s so easy to catch. While it may sound counter-intuitive to show students how to plagiarize, it will show students that you know the all their tricks (which they probably already knew already). This strategy of talking about the methods of cheating might prevent cheating in the first place.