David Bernstein, Orin, and David Lat offer some useful tips to law students inteviewing with firms. I would add one more: do whatever you can to convey the impression that you're enthusiastic about taking a job with the firm (without lying, of course). In my case, I lost out on many offers from major DC firms during my first fall interview season because they suspected, based on my resume, that my real goal was to become an academic and I didn't make much of an effort to convince them that I had a genuine interest in practicing law at a firm.
The next year, I made a special effort to display more enthusiasm and study up on the fine points of the various firms' practices, and did much better. The academic thing still cost me some offers because, well, I really did want to be an academic and the firms weren't stupid enough to completely ignore that! But it was much less of a problem once I showed them that I had a real interest in their work too.
Even if practicing at the firm of Larry, Moe & Curley is not really your idea of a dream job, when interviewing with them try and act like it is. Perhaps it's an obvious point, but I'm one of many students who didn't get it until chastened by painful experience.