Address plagiarism before it occurs. Include a plagiarism exercise in your class--Indiana University has a nice website which includes an overview, tutorial, and test.
Provide written guidelines for writing papers. Include information on academic honesty, plagiarism, paraphrasing, direct quotes, summarizing, and when and how to cite sources.
Point students to good resources: Purdue University provides excellent information on paraphrasing, quoting, and summarizing, plagiarism, and properly citing sources.
Include a plagiarism statement in your syllabi.
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 Turnitin. Require students to submit their drafts and papers to Turnitin.