A perennial problem, no doubt, is plagiarism. My approach to this has been to point out my university's academic integrity policy, and, as we are required to do, include it on the syllabus. I also mention it the first day. I've wanted to avoid making a big deal of this issue, especially on the first day, but I'm now rethinking this.
I have had students simply turn in entire papers that they downloaded off of the internet. These cases are fairly easy to deal with. I have more trouble, however, when a student has a sentence or two lifted from a website that I find via a quick Google search, but the rest appears to be their own work. I also wonder how much our students understand about what constitutes plagiarism, as I watch my own children use the internet to look up words, historical figures, and so on for their assignments. I've decided to start using SafeAssignments, a service our university uses to detect plagiarism. However, a more important question in my mind is how do prevent this in the first place? I know some professors spend a day or two crafting a policy in cooperation with the class. Has anyone tried this, or something else that has been effective?