Interviews are about matching to a lab, its activities, and the mentor. Read over some of the articles from the lab or advisor, but don't kiss-up by memorizing details. Ask questions about ongoing research that demonstrates that you will be a productive and enthusiastic researcher when you get there. Ask lots and lots of questions about the program to demonstrate that you are interested (for many of the interviews, it will be you asking more questions than answering). Remember that the students and staff will report to faculty how you acted with them too, so stay 'on' for your vsiit. Other than that, plain old social skills should do the trick. If you don't match at a site, it may not have been the site for you. You want to be happy for your 4+ years in grad school, so let the match work itself out and trust the process.