Know enough about each firm to be able to confidently explain why you are interested in working there.
Be professional and personable with your host, during presentations and during the interviews.
Dress for success.
Prepare solid questions about the firm; culture, keys to interviewers success, etc. and build more questions based on their answers for a good conversational flow.
Provide good nonverbal communication throughout the interview; good eye contact but don't make it weird, sit upright in your chair but lower your shoulders to seem comfortable.
Calm your nerves, everyone will be a little nervous, but try to seem controlled and calm throughout the interview and it will better help you converse and answer questions.
If the firms do presentations between interviews be sure to act engaged, take notes over things that may make good questions during the interviews, and ask good questions at the end of presentations, most candidates won't ask questions so this will give you an opportunity to stand out.
Additionally, I don't like to show up to anything empty handed, bring a nice leather portfolio with a copy of your transcripts, resume, a note pad and an ink pen to take notes. (Sometimes I cheat and write clues to my questions/answers for the interview on the bottom of my note pad and look over them discretely if possible to keep things fresh on my mind.)