Education is the best prevention.
These are resources that can be helpful in explaining to students what plagiarism is and how they can avoid it.
1. Plagiarism Explained by Common Craft.
2. The Purdue OWL website is the number one place I refer students and parents to for questions not only about plagiarism, but also for questions about all parts of the writing process.
3. Plagiarism.org, produced by the same people that produce the commercial plagiarism detection software Turn It In, has a free learning center for students and teachers. Plagiarism.org's learning center includes tips about avoiding plagiarism, definitions of plagiarism, and explanations of when you do or do not have to cite a reference. Plagiarism.org also hosts two recorded webinars addressing the topic of plagiarism in schools and how teachers can educate their students about plagiarism.
Tools for detecting plagiarism.
4. The first thing I do when I want to check a student's work for plagiarism is to do a quick search on Google. If you notice that a student has strung together some phrases that you don't think they've written, put the suspected phrase inside quotation marks and search. You may want to search on Google as well as on Google Scholar.
5. The Plagiarism Checker, created as a project for the University of Maryland, is an easy-to-use tool for detecting plagiarism. Simply enter a chunk of text into the search box and the Plagiarism Checker will tell you if and from where something was plagiarized.
Update November 2014 - Links to Plagiarisma were removed after a reader informed me that the site was selling plagiarized copies of essay. Ironic, don't you think?
7. Paper Rater is a free service designed to help high school and college students improve their writing. Paper Rater does basic spelling and grammar checks, but the real value of Paper Rater is that it tells students if their papers have elements of plagiarism. Paper Rater scans students' papers then gives students an estimate of the likelihood that someone might think that their papers were plagiarized.
8. Plagiarism Checker.com works just like many similar services. To use it, simply type or paste text into the search box and Plagiarism Checker will tell you if and from where something was copied. (Note: the name is similar to #5 above, but they are produced by different organizations).