My experience with student plagiarism has taught me that the following conditions create plagiarism: 1) student's lack of knowledge and/or confusion about how to use resources, cite properly; 2) No, or little, experience with library use, inadequate research skills, the rigors of academia (i.e., writing a paper based on research takes time and effort; 3) redundancy on the part of the teacher.
The first two can be taught; the final one cannot, because it relies on the teacher's willingness or ability, given his or her strictures, to change his or her approach. Making every paper a new challenge, never relying on old paper ideas, and teaching students how to quote properly and cite sources properly, are the beginnings of how to avoid plagiarism.
I taught for years at a university that focused heavily on plagiarism concerns. However, I never once had a student plagiarise, and I can tell you why: each paper was written beginning from personal experience, and added to with research. The nature of the assignments make students want to do the research. Now, this is not hard to do, but it does require approaching teaching writing from a different perspective than the kind of rote learning that sadly, promotes the lazy desire to fill in the blanks on a piece of paper for a grade. Motivation, and rhetorical analysis, is key to this method of approach. It's not complicated, and it's not hard; it's just not an approach most in English departments are used to, I found.