Who would have thought that great advice to teachers for preventing cheating might come from Turnitin, the company that earns money buy selling its service to colleges and professors hoping to sniff out plagiarism?
This graphic from Turnitin shows the sources of students used when plagiarizing and, at the bottom, provides some tips on developing "plagiarism-proof assignments". There is no such thing; if someone really wants to cheat, they likely can find a way. However, the tip to break up assignments, with various due-dates, is a big deterrent to students purchasing papers. I have also heard of professors building toward major assignments and projects with smaller, discrete assignments. For example, assigning students to find sources on a topic or research question. Later, asking students to write an abstract of several articles they found for their source assignment. Another assignment might build on those assignments into a literature review. Again, organizing assignments like this will not prevent cheating in all cases, but it can help by making it much more difficult for students to cheat.
The Chronicle has a recent article about cheating as well. Two points really struck me - some students feel very uncertain in knowing when they have plagiarized and when they have not. I was surprised by this - wouldn't you know if you copied and pasted someone else's words into your paper and didn't cite them?
The other point was that students are not taught the more complex skill of engaging with texts and summarizing main points. The Chronicle article interviewed someone from the Citation Project , which studies how students cite sources in papers. The aim, their website says, is to help educators develop "best practices for formulating plagiarism policies and for teaching rhetorically effective and ethically responsible methods of writing from sources." From the studies of student papers done so far, they find that students often pull quotes from the first few pages of articles and don't engage deeply with the overall arguments. This is useful information in understanding how plagiarism happens and how to improve student writing.
Finally, in the comments on the Chronicle article, someone linked to this excellent resource (pdf) from the University of Wisconsin that describes the difference between quoting, paraphrasing, and citing sources.