Purdue Owl
This site gives an overview and some exercises and it has links to style pages (MLA, Turabian, etc). It is geared to high school and university level but will have ideas that middle school teachers can adapt to their students.

St. Martin's Tutorial
This site form the University of MA at Amherst has a well developed set of exercises with quizzes to learn about summarizing, paraphrasing, using integrated quotes, and when to cite. It also has information about how to manage a research project. HIgh School and university level but a good resource formiddle school teachers.

Article about plagiarism proof assignments
This is an interesting blog entry about creating innovative "plagiarism proof" assignments. Will spark some good ideas.

Strategies for preventing plagiarism
A site from Colorado State University which includes a tutorial and information about plagiarism as well as about why students plagiarize.

Plagiarism proof assignments
A site from Minnesota State Colleges and Universities. With activities, quizzes, and a video of a seminar on plagiarism.