1) Be confident and be yourself.
2) Honestly look at the interview process not just as an opportunity for them to "assess" you, but also for you to check out the school and see if it's a right match for you as well. It's a learning experience; it can be a lot of fun if you go about it with a sense of curiosity & excitement. 🙂
3) Although I know this is a debated topic, I believe it's advisable to write a thank-you letter afterward. Try to pick out something unique from the interview so that it doesn't seem like a form letter, i.e., don't just say, "Thank you for interviewing me, I think the school is great, etc." Add something that makes you unique!
Hope this helps! 🙂