My recommendation, which should be taken with a grain of salt since I haven't gone through this process yet, would be to write a very very specific LOI indicating why you love the program, how your personal qualifications would add something to the class, your excellent experience at the interview, and how if you are accepted, you will absolutely drop any other waitlists or acceptances at once. I am not a fan of calling all the time since I think it probably bugs the Admissions offices, but a few well-timed and succinct phone calls won't hurt.