In reality plagiarism is synonymous with copyright infringement. We live in a copy and paste world today and it can be very difficult for students to understand the concept of using others' information as if it were their own.
I think the best way to teach students is to lead by example. What comes to mind this instant is the lesson that 4th graders are learning in the computer lab right now, taught by a para-professional, about how to copy and paste. They will be doing reports in class and need to have photos attached as part of their grade. And they are learning to copy and paste them from the internet. This would be the perfect time to introduce students to copyright infringement and the importance of using sites that allow for copy and paste. Unfortunately, this is totally overlooked.
To help curtail the events of plagiarism, I purchased the World Book Online this year for our students and it has a built in citation piece that will help students to identify their sources more accurately. As digital literacy increases, so does the need to teach and prepare our students. It is never to soon to teach paraphrasing and summarizing. This can and should be done early on in elementary school and in fact, I believe this is addressed as part of the Common Core Standards. Students in middle school are not sure what the difference is and to be really honest, sometimes I'm not sure that teachers do either.
Students need a lot of examples and plenty of opportunities to practice.