Suppose a student is faced with a choice. Spend several hours reading, researching and pondering the decline of the Byzantine Empire, then another hour or two applying that newfound knowledge and careful, critical thinking to the writing of a 500-word essay -- or copy and paste someone else's work on the same project. Done and done. Now the evening's free to watch Conan and play a little Gears of War 3.
It's a tempting proposition. The student knows he/she is in the wrong to take such a shortcut, but when you combine the growing laziness that seems to be sweeping the country with most teenagers' short attention spans and need for immediate gratification, it may be hard for some students (those with a flexible sense of morality) to resist. It's that plate of cookies on the counter that you know you should keep away from because you're determined to stick to your diet ... but they're right there.
There are ways of catching offenders. If a teacher finds a particular passage somewhat suspect, they can plug selected phrasings into a search engine and see if they come up with a match. If they really want to do their homework, there is plagiarism protection software available, but we'd imagine there are better ways teachers could be spending their time than filtering hundreds of student papers through such a program.
The fact is that the technology is there, easily accessible, and not going anywhere any time soon. The best shot we have at discouraging internet plagiarism is to go straight to the source of the problem -- those seeds of mischief from which the trees of criminality grow -- and appealing to their sense of right and wrong. It's in there somewhere.
It's important for educators to accept that this is a real and widespread issue, and to bring it up themselves in the classroom rather than just sweep it under the rug and hope no one goes snooping for it. Make clear and explain to students just how appalling a transgression the act of plagiarism is, and why. If that isn't enough to deter some would-be word thieves, spell out how severe the penalties may be if they are caught, and tell them that you'll be performing periodic, random searches (even if you're bluffing).
Last, throw a little guilt trip their way. Remind your students that they are intelligent, mature young adults who are about to be trusted with important privileges and responsibilities out there in the real world, and they will be expected to behave professionally and with the utmost integrity at all times. A casual mention that you personally will be disappointed in them if they plagiarize couldn't hurt. Nobody wants to let down the teacher.
If all this fails and you discover that a student has ripped off some other writer in cyberspace, slap a big fat "F" on their paper. Then, when they ask why, simply tell them that you saw another teacher give that mark, and copying what they did was just easier than grading the paper yourself. They'll be certain to appreciate the irony.