I would definitely advise sending hand-written thank you notes by mail. Check with the program coordinator; she may be able to distribute individual notes by intracampus mail if you send her all of the thank-you's in one large envelope.
Just wondering if I should send the thank you letters by e-mail or mail. Any input would be greatly appreciated.
I'm sure there is no one right (or wrong) way, but I was advised by our program director to send one typed post-interview thank you letter (doesn't matter how) to the program director (plus one to the coordinator if you wish. Great suggestion benit from the previous post!). He noted by snail-mailing multiple thank you notes, they would probably all end up in your file anyways, making your folder unwieldy and exposing the fact that you used the same format (if not the same actual letter) for all the interviewers.
However, as with all things, it's totally individual. If you really hit if off with a particular interviewer, I'd send another thank you note (email is probably best, to bypass the secretary, who'd probably just drop it into your file, unless you verify that she/he will forward it along).