One of the most effective strategies for preventing plagiarism (intentional or not) is to discuss the concept of plagiarism with your students.
Early in the semester (maybe the second week) ask your students to describe or define "plagiarism." This could be done in a Write to Learn activity that precedes a class discussion. You may find that student conceptions of plagiarism are very different from academia's or your own. However, discussing these conceptions with your students provides the opportunity for you to clarify misunderstanding and to reinforce why plagiarism is unacceptable.
An open discussion also allows students to engage with the concept of plagiarism instead of merely having it imposed upon them; it helps create an atmosphere in the classroom that is conducive to learning and academic integrity. After discussing the concept, everyone can be clear about the expectations concerning plagiarism. That way, a student is never caught off guard if the issue arises, and, moreover, students can be held accountable for maintaining academic integrity since they are aware of the expectations set for them.
You can use the following Write to Learn to jumpstart your discussion about plagiarism:
Take 5 minutes to answer the following questions:
What is plagiarism?
Why is plagiarism inappropriate?
Why might a student plagiarize?
What do you feel the consequences for plagiarism should be?