I think the common interview questions can be easily found on the internet or at any law school career services department. That's the easy part. Everyone will be prepared for those questions, so you need to set yourself apart by going the extra mile.
When I interview attorneys for a position in our firm I am the most impressed by applcants who have quite obviously not only been to our website but who have REALLY spent some time there getting to know what our firm is all about, who we are and who our clients are. It is always great when they remember something about your bio that is unique.
Also, you need to know exactly why you want THIS job, not that you are just searching around for the highest paying job or that you just happened to answer an ad. We want people who want us!