This is a good idea:
Every semester, about this time, I identify students in my intro classes who are doing reasonably well, seem interested in philosophy (based on class participation, conversations in office hours, or written work), and are not graduating in the near future. I email all of them individually (although of course the letters are somewhat repetitive) and suggest that they take more philosophy, and why. I encourage them to come talk to me about which classes they might be interested in.
This seems to be pretty successful. Almost all of the students I email come talk to me, and I have good evidence that the majority take more philosophy classes. (Of course, I don’t have access to course rosters in the worlds in which I didn’t do this)
There is some risk that contacting students in this way could favor certain groups over others. In my experience, without any deliberate preference towards any group (except as noted above), at least 50% of the people I’ve emailed are women and the lists reflect the diversity of my classes in other ways as well.
That’s Brian Talbot (Washington University in St. Louis)—who says he doesn’t deserve credit for the idea, which he has seen others discuss—with a timely reminder of some of the benefits of reaching out to students individually to encourage them to take other philosophy courses.