Plagiarism is a vile offense. Here are some tips on how to avoid it.
1. If you take decent notes there is less chance that you will inadvertently plagiarize. Make sure that you always put quotations in quotation marks when you take notes. This minimizes the risk that you will later transcribe the notes into your own essay, thinking that it is your own work.
2. Err on the side of caution.
3. When in doubt, ask your instructor.
4. Remember to cite ideas, as well as quotations.
5. When you're trying to figure out whether or not to cite something, it can help to remember the point of such rules:
a. To give credit where credit is due.
b. To bolster the argument (for example, by citing work that defends a point at greater length).
c. To indicate the extent and kind of the writer's research.
6. If you read something which you came up with independently, either:
a. note that you came up with the idea independently when citing; or,
b. just accept that someone got there first and give them the credit.
By the way, if a friend is tempted to plagiarize, you can offer him/her the following arguments:
1. A plagiarist's chances of getting caught are actually pretty good. Professors and graduate students have usually read an astonishing amount of secondary literature, and in more than one language. Also, computers are making it much easier to catch plagiarism these days.
2. The consequences for plagiarism can be fairly brutal. It's a better bet to fail the paper than the plagiarize.
3. Your friend is cheapening the value of your degree. You're going to pay a lot of money for that degree.
Here is one way to cite sources:
Young, Christopher. 'Wither the Weather?,' Canadian Journal of Finnish Studies 5 (1999), 13-56.
Young, Christopher. Reflections on Mirrors. (New York: Little League Press), 2001.