I'm a PhD student and have interviewed applicants to our program the past few years. Here are the specific things that come to mind from interviewing people:
Try to remain calm and use coping skills to control your anxiety. Nerves are expected, but if you're literally shaking with fear, it raises doubts on how you're going to be able to handle stressful clinical interactions in grad school.
Have an idea for a research project (e.g., research questions, population, etc.). But be careful about proposing something super expensive that isn't feasible (e.g., fMRI research if your PI doesn't have access to that).
Don't advertise (even to grad students!) that you already got in somewhere and think you'll choose that school because you love it. It's rude.
Don't steal your host's research idea and then propose it as your own to the PI during the interview (yep, one of the applicants I hosted did this...and my PI saw through it quickly).
So basically - if you remain relatively calm, have a feasible research idea, and don't do anything blatantly rude/unethical, you should be good!