I just discovered this forum this morning, while mildly panicking preparing for my first formal interview, with Emory. I tried to do all the standard prep things (preparing answers for why I wanted to attend there, why I thought I'd be a good fit, what their department's personality was, what books the professors had written, etc.), but I think that the most helpful prep I did was to a) relax, and prepare articulate answers about myself. For instance, in the notes I made myself to prepare, I made sure to write out the names and authors of books I'd been reading and thinking about lately, issues that have been interesting me of late, etc. It meant that I could use those as concrete starting points to talk about things that I'm genuinely excited about, which is definitely important.
Also, for what it's worth, both this and the other (informal) interview I've had were really enjoyable experiences, with professors who wanted to talk about interesting things in my field, not just grill me for the Right Answer. I'm sure that the latter kind of interview happens too, but I don't think it's the norm.
Best of luck!