Students may be in a hurry to complete an assignment in which they are not interested. They may be lazy or they may simply not know what plagiarism is -- but plagiarism is serious business. It can get a student expelled from a university or cause a person to be dismissed from a significant position, as might be demonstrated in these plagiarism facts and stats.
Perhaps the best time to teach about plagiarism is the first time students are taught to write a research essay or book report -- especially if they are also taught how to do research on the internet.
Another helpful site with numerous free resources is sponsored by Turn It In, a subscription-based service which many university professors use to check for plagiarism in the work of their students. And at Plagiarism.org, one may find podcasts such as Teaching About Plagiarism with Help from Dr. Seuss
There are lesson plans for Teaching Students to Avoid Plagiarism, and while not specifically touted as such, I think Andrew Pudewa's Institute for the Execellence of Writing (IEW) method of summarizing articles is rather helpful in this regard.