One important thing that no one really tells you, probably because of how lame it sounds, but: Just be yourself. Yes, most of it is what's in your book, the other part is your personality. My former boss, when he was interviewing me, looked me straight in the eyes and asked me if I was a jerk. I kinda half smiled and said no. But he was serious. This can be a very stressful field and when things get crazy, no one wants to have to work with an a$$hole. So, just let as much of your personality through as possible and be genuine and personable...Things like: If it's Monday morning, Ask how the recruiter how her weekend was...Maybe you'll find out that you both went to the same design show over the weekend...You just want them to see you as another breathing living human being and not just another mini book. Anyway, good luck!
Oh, also, be ready to explain how you developed your concepts for any of your ads...Some of the spec ads I've done, I did a while ago and I'm always caught off guard trying to remember how I got to the final idea. But yours should still be fresh in your head.