Know your research inside and out. Nothing looks worse than not knowing something in your publications when you're an author on it. DIGA has a set of interview questions as well that you may want to look at:
http://www.derminterest.org/Links/DIGAResources.aspx
Be prepared to answer why you want to go into derm and make sure your app proves it. That's the biggest pet peeve of derm faculty, someone says, for example, they want to be able to help the underserved with their specialty, but nothing in their CV proves that. It looks fake and contrived and faculty catch onto it.
Also, be aware that you're being watched by everyone: the secretary, the program coordinator, residents, fellows, etc. Not to get you nervous, but at least act like you want to be there. You'll be amazed the things people do and how that filters back up to program faculty, bc the applicant completely let their guard down.
Oh, and make sure you've fully researched the program where you're at -- meaning who you're interviewing with, what their interests are, what that program offers, etc.