How to reduce student plagiarism? During the last term, I recorded at least 50 cases of student plagiarism. The most common cases were students copying and pasting paragraphs verbatim from various Web sites, assembling them together, and calling it their essay.

I took what I thought were sufficient steps to inform students of what was not allowed:


I posted the rules in the syllabus, on the course Web site, and listed relevant rules in the instructions for larger projects.
I issued spoken warnings in class regularly, occasionally showed some examples of such submissions, and also showed students some of the steps I took to catch the plagiarism.


I also set what I thought were strict enough consequences so that students know it is better to do nothing at all than to cheat:


20% grade loss (from their entire grade) per infraction, no matter the value of the assignment (most assignments were only worth ~5%).


Note, these are policies I established from the very first day of the class, and carried through the whole term. Yet, even in the final weeks, I continued to catch copied work and failed a lot of students.

What further steps can I take to reduce this problem?
It might be helpful if you can tell us what field you are in, or more specifically what kinds of class you are teaching. What is the subject, and is it a big lecture class or a small section, etc? Catching 50 plagiarists makes me think you are teaching big lecture sections. 

Here are two things I do: First, I have a very harsh plagiarism policy. I automatically fail anybody I catch plagiarizing. This raises the stakes.

Second, I consciously try to design assignments that are hard to plagiarize. There are a variety of ways to do this. For instance, you can give fairly specific assignments. Don't say: "Write a paper about Shakespeare" but instead "Write me a paper on the role of ghosts in King Lear and MacBeth." This doesn't make cheating impossible, but it makes it harder to google and get a prefabbed paper.