Well, other than preparing my clothes and my documents (if any) and getting there in time, I think about the company and the position I'm interviewing for. I think about what I can bring to the company, why I would be a good hire for them, and what I can learn while there.
I also think it's important to have researched the company so you know what they do, who they work for (if applicable, ie. an advertising agency), who and how many are in the company, and think about why you want to work for them. Why them? Is it just because a position is open there or because they're located near you or is it because you've always wanted to work for an advertising company?
Do you want to be a receptionist or a salesman there? Why? What makes you a good fit for them? What can YOU offer, specifically? Hard work, honesty, reliability, talent and/or particular skills, experience?
That kind of thing.