Define plagiarism for your students. Explain that any time they claim someone else's ideas as their own, whether from omitting a source or copying text verbatim, they are plagiarizing. Remind them of your school's policy on plagiarism. Tell them that it is possible to commit plagiarism accidentally by forgetting a citation or failing to paraphrase properly, and that it is important for them to pay attention to the plagiarism lessons to avoid this.
Engage the students in a discussion about intellectual property rights. Help them to understand why plagiarism is wrong. If they can come to understand this through discussion, they may be more likely to internalize the idea.
Show students how to cite sources both in-text and in their bibliographies. Provide a model bibliography as a reference. Do not assume that they are familiar with your preferred format for citations, especially if you are teaching a freshman course or your college has a high number of transfer students. If you teach middle or high school, assign students worksheets and quizzes to practice in-text and bibliographic citations to assess their mastery of this skill before moving on.
Tell students that they must paraphrase their sources. Explain to them that paraphrasing is rewriting a quotation entirely in their own words, not simply replacing key words with synonyms. Ask them for examples of movies or books that "rip off" other movies and books, and tell them that changing a few words into synonyms in their essays is like changing a character or two in a movie but leaving the plot identical.
Remind students that if they don't paraphrase properly, you can't be sure that they truly understand the information. To illustrate this, give them a piece of difficult text, such as an excerpt from "Finnegan's Wake." Have them replace a few words with synonyms and ask if they understand the meaning of the text any better now than they did before. Explain that when they do the same with their sources, you can't know whether they understand the information or not. Have your students paraphrase texts on their reading level to assess their understanding of this skill.
Show your students an essay purchased from the Internet or copied from a reference website and grade it together as a class. Because these essays are generic, they will rarely meet all of the standards on your rubric and thus receive a poor grade.
Collect papers in stages. Have your students turn in research notes, outlines and rough drafts before turning in final drafts. This will not only let you catch any plagiarism before the final draft, but will also encourage students to start working on their papers early and thus avoid the last-minute panic that may tempt them to plagiarism. It will also improve the quality of their papers by helping you to catch any students who are writing off-base essays.