I want to bring attention to the fact that not all students plagiarize with the intent to do so. I think students are not always taught proper citation rules, and can make unintentional mistakes when they are writing, especially if they are still in High School or just starting out college.
We all know that we have to cite anything that we directly copy from someone else. But most students don't know that they also have to cite information that they re-phrase themselves. Most of my students make this mistake in my classes and I take every opportunity to remind them about it.
There are cases where a student has taken an entire paper off of the internet and thinks that the instructor won't notice. I can tell you that we always notice. If you are having problem writing a paper, always ask the instructor for help. If you decided you will write a paper the night before it is due and failed, try and ask the instructor for an extension. But never plagiarize or copy other people's work!
As for citations, remember to cite everything that is not of your own thinking. If you're not sure about it, cite it! These are my suggestions to students who want to avoid academic dishonesty.