Just relax. Definitely don't call and beg. Be happy that you've had interviews - this means that you're in range to get in. Many schools accept half (or more) of who they interview, so statistically, 2 * 50% = 100%, your chances are excellent. You've done your part - you've applied and interviewed. This is no time to be a psycho and call them hourly. Personally, I think your fate is determined soon after your interview because that's when interviewers provide feedback. Maybe you could write a letter describing how much you like the school, but only do it if you have very specific compliments. If you want to do the letter, maybe do a post asking peoples' opinions. I had one school where I did nothing extra, and got accepted. I had another where I did lots of extra stuff, and never got off the waitlist...