Establishing the consequences of plagiarism early in the semester is the best way to start, but you also must enforce them.
Of course, we only want to confront a student when we have concrete evidence of plagiarism. But sometimes the first step is simply talking with the student. Dornan et al. suggest questioning students about methods used or understanding of the citation process (147). Approaching students from an inquisitive stance instead of a condemning one will keep students off the defense as much as possible and will open the lines of communication so that you can determine what/how much they did not understand and where the line of integrity may have been crossed.