Tips to Prevent Academic Dishonesty
It is important that instructors put a statement regarding academic dishonesty in their syllabi. This informs students that you are aware that cheating occurs and that you plan to pursue all incidents of academic dishonesty. Just as importantly, it allows you to formally clarify what sorts of grey-area actions are or aren't acceptable in your course (e.g., collaborative work). You may want to include a statement about your policy for handling cases of suspected cheating (e.g., "Any instance of academic dishonesty will be reported to the Academic Judiciary Committee and can result in an F for the course.")
Cases of plagiarism often involve students improperly using Internet sources. If you allow students to use internet sources in papers for your class, tell them to include the URLs for those sources in their bibliography. You should warn them that copying (or closely paraphrasing text) text or figures from a website without citing it and placing it in quotation marks is plagiarism. It is no different from doing the same thing with a printed source. Professing ignorance of this rule will not be accepted as a legitimate basis for appealing an accusation of academic dishonesty.
If you suspect a student has plagiarized an internet source, the simplest way to test this hypothesis is to type a few key terms related to the topic of the paper or phrases from the suspect paper into "Google" or some similar search engine. (This is how the students find the sources in the first place.) If the student has plagiarized an internet source, you will probably find it in the first dozen "hits".
Approaches to Prevention:
An excellent discussion of plagiarism can be found at http://www.indiana.edu/~wts/pamphlets/plagiarism.shtml