Get a good night's sleep, eat something before going to interviews (even the ones that say they will have some breakfast stuff - don't go in and stuff your face, maybe grab a juice or a coffee), and just be the best version of yourself you can muster at 8 am.
I haven't done any "prep" other than to look over the info they sent, scan their website, and make note of the interview day - look over the names of the interviewers - get a feel for the flow - but I don't do anything beyond that, like googling them and their practice and research interests. The interviews aren't that long (most common length is 30 mins, and I have had a couple of 15 minute 'speed dating' style interviews) and they do most of the talking, and they spend a lot of time selling the program. A big chunk of the day is group presentations, some breaks, lunch with residents, walking tours. It is exhausting to be "on" for the entire day, but that is your challenge and goal, to be on your toes, ready with a smile.