Check out this interesting article by Professor Jeff Karon (University of South Florida) on plagiarism and how to prevent it.
Here are some other tips you might find useful:
Be proactive in preventing cheating on papers and assignments. Let your students know that you expect them to do their own work and that cheating will be punished in your class.
Put your university’s policy on your syllabus. You should also be familiar with your university’s procedures on reporting cheating so that you follow the appropriate steps if this becomes necessary.
Define the term plagiarism for your students. Many of them do not understand the difference between citing and copying someone else’s work.
Give specific and timely topics on papers so that students cannot simply access the internet for commercially produced papers. Thus, a paper on sexual harassment in the workplace is easily available for sale. A paper on sexual harassment policies in local companies in your town is probably not.
Have students turn in outlines and/or first drafts of their papers to you earlier in the semester and then have them attach those drafts to the final paper when it is due.
Require students to turn in copies of all sources cited including websites, journal articles, and pages of books used.
Read the Bibliography. Dates should be required for interviews and websearches. If these dates do not fall in line with what is reasonable for the semester due dates, that should raise a red flag (tip submitted by Timothy Johnson).