Be yourself, be relaxed, and be friendly.
Interviewing is very similar to an oral argument. Go in prepared with a rough outline of about five things you would like to discuss. Tailor these topics to the firm. The key is to bridge things from your past experiences with things that the firm does or qualities the firm has. This way it doesn't seem forced, but they are still learning about you, and they are learning precisely why you would be a good fit with their firm. For instance, you are interviewing at a large firm. You can talk about how you grew up in a small town and went to a small high school, and then when you went to college at such a large university, you learned just how many more opportunities it could offer you. Blah, blah, blah.
It is equally important not to be rigid with your agenda. Again, like an oral argument, you gotta be ready to roll with the punches and go where the interviewer takes you. If you can take their question and tie it back into one of the topics you wanted to discuss, then you are doing great! If they want to take it somewhere else, don't worry about it. If they just want to talk about themselves the whole time, that's okay too. Just seem interested in what they have to say, and when you get a word in edgewise, try to make it count. For instance, "It is funny that you should say that. I have had similar experiences in my past... blah, blah, blah."
Finally, be prepared to ask questions. Every interviewer will expect you to have questions, and you look like a dumbass if you don't know enough about to firm to know what to ask. I found this particularly difficult after like the 6th interview on a callback, especially when I had already asked my questions of previous interviewers. On callbacks, take notice of the seniority of the person interviewing you. For instance, if you are talking to a first or second year associate, it is probably okay to ask something like "How hard is it really the first couple of years?" or "What kinds of significant responsibilities have you been given as a young associate?" When interviewing with a mid-level or senior associate, ask about the firm's partnership track, competitiveness, and quality of life. When you are interviewing with a partner, you should ask more strategic questions about the firm's philosophy, and direction.