I started out sending handwritten thank-you notes to every person I came in contact with: the director, the administrative staff, and every interviewer and faculty member I met with.... but after several interviews grouped together right around the holidays, I just gave up. I e-mail directors or interviewers who I particularly clicked with or may want to work with, and that's that. I feel a little guilty, but it just didn't seem worth it to write dozens of thank you notes at a time. After my next interview, I plan to just e-mail a quick thank-you as soon as I get home.