There are hundreds of pages on the web dedicated to helping teachers fight academic dishonesty in all forms.  Here is a short list of pages to get you started.

Developing Assignments

Anti-plagiarism strategies for teachers
Written by Robert Harris, the author of the Plagiarism Handbook, this site gives a very complete overview of how to develop assignments that may reduce the urge to cut and paste. Also gives a list of clues that may tip you off to a plagiarized paper and ideas where and how to look for the original sources of suspect papers.

From the School of Education at Indian University at Bloomingdale a tutorial that you might find useful for yourself or your students.

Most of the sites dedicated to faculty struggling to develop assignments list the same suggestions.  One that is rarely mentioned is one employed by a faculty member at Newberry College.  He required one of his upper level classes to write a research paper based on seven books that he regarded as the core readings on the subject. The class was small and to ensure equal access to the resources, the books were kept on reserve in the library. The professor was intimately familiar with the books and knew immediately when the students were not acknowledging the sources properly.

REALLY interested in plagiarism? There's a new journal (Jan. 2006) devoted exclusively to the topic. Called Plagiary: Cross-Disciplinary Studies in Plagiarism, Fabrication, and Falsification, it is produced by the Department of English at Saginaw Valley State University.

Evil House of Cheat and Co.

Term Paper Mills
The librarians at Coastal Carolina collect the web addresses of every term paper mill they can find. Everyone needs a hobby.

Software

Turnitin
     The most widely used of the programs -- ABAC has a site licence for this product; contact Dr. Amy Willis for a username and password and for instructions for use.  Students submit their papers electronically to the Turnitin people, who then compare the assignment with a huge database of papers posted on the web and other on-line resources.  The professor receives a report of the result. NOTE:  Turnitin does NOT compare papers with all of the on-line full-text databases -- common sources of plagiarized material. 
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PLAGIARISM.COM
Glatt Plagiarism Services offers interactive software that teaches how to  avoid plagiarism, screens student work for plagiarism, and suggests a number of self-detection procedures.

Noodle Tools
Noodlecite software is designed to help students cite what they find accurately and painlessly. NoodleBib is a free tool that will help students produce accurate citations. Another place on the web that will help students produce citations is The Citation Machine.

The Method
If you bring a suspect paper to the library and ask for help in finding out what  the original source(s) of the paper might be, this is the process that the paper undergoes.