If there are copies, it is easy to check references for accuracy and correct documentation, and it helps students understand that they will be at least spot checked. You can run the papers through the site and it will check across the web for plagiarized sections. I ask students to locate the plagiarism among these samples by comparing them to the original. As someone from a department other than English, I have noticed that the type of plagiarism most students commit is unintentional and resembles the last sample paragraph. Many seem genuinely surprised when I tell them that the last sample is plagiarism.
Most students seem to know how to cite when quoting directly from a source.
It is in paraphrasing that they run into trouble. Inevitably, there are problems with proper citation. I have found that students are genuinely interested in discussing plagiarism and how to avoid it, and the problem seems to resolve itself quite quickly once they are aware of it and the fact that I am on the lookout for it. I also let students know that I spot check sources, and so do professors and graduate students across the university. The final thing I do with regard to plagiarism and proper citation is tell students that if they are ever unsure about proper citation, to please come ask.