Uriah Kriegel has some clever advice for fighting plagiarism:
Every assignment I give is a two part assignment. The first part is to read your institution’s academic code of honesty plagiarism section. The second part is to write a paper. Failure to fulfil one of the two parts is failure to do the assignment, therefore deserving of a failing grade.
What this does is neutralize the “I didn’t know it was plagiarism” line. A student who didn’t know it was plagiarism did not fulfil the first part of the assignment and therefore fails.