Tell your students that you check for plagiarism, then follow up. A good way to deter plagiarism is to develop a reputation for intolerance.
...your assignments thoughtfully. Require process steps. Break the assignment into pieces, and provide due dates for each specific piece. Approve the topic, require drafts, require an annotated bibliography, current sources, and specify the style (ACS, AMA, APA, ASA, Chicago, CSE, MLA, Turabian, etc.) that students should use.
Be very clear with your expectations. Require specific sources for your papers--one webpage, scholarly journal articles, scholarly books, a reference source, perhaps an interview. Make sure the sources students use have been published recently. Consider requiring the students to use a source provided by you.
Use Unicheck. Require students to submit their drafts and papers to Unicheck.
Utilize Word document properties--view authorship, date created, and time spent editing.