Educating students about the types of plagiarism can be very effective in preventing plagiarism. Many students don’t even realize that they are plagiarizing.
See http://www.plagiarism.org/ to learn about the different types of plagiarism and for tips on preventing plagiarism.
You might also be interested in an article I recently read titled, “Designing Assignments That Discourage Plagiarism.”
If you do use one of the plagiarism detectors, you might consider compiling the results and sharing the data with your students. Jamie Mahmoud and I did just that a few years ago with our Freshman students. I actually searched Google directly and found that out of 83 graded assignments, 61 students plagiarized in some way. I’ve attached the presentation that I put together for the students.
See > English09CitationData