My one regret is not doing enough mock interviews! Practice, practice, practice. It is NOT enough to just have a general idea of what you're going to talk about. Even if you normally consider yourself to be a well-versed and generally social person, you want to have an EXACT idea of how you will answer questions, and practice such that when you are asked the question you will have a well rehearsed answer prepared and won't go blank. Note that there is a very fine line between being well prepared and sounding like a robot--you still want to maintain a natural and conversational tone and the hard part is maintaining that balance.
I received 4 interviews and sadly only 1 acceptance and that acceptance was after the only interview I actually went through a mock interview with with my school's pre health advisor beforehand (it was also the only interview that was an MMI--I actually found this format easier to work with, let me know if you want me to elaborate). I think it helps to work with someone you know only on a professional level, like your pre health advisor, or a professor or even a dentist--simply practicing with a family member or a friend is not sufficient to prepare yourself--it's too informal and doesn't replicate the actual interview well. The great thing about mock interviews is the feedback you get--for example I got tips on how to improve my body language, which was honestly something that I hadn't even considered while answering the questions.
Also note that even if you prepare well and have an excellent interview, there is always a chance that a school will still waitlist or even reject you. It's not possible to know what each and every school is looking for in their class and sometimes it's often just a case of them wanting to have a well-rounded class. You might get waitlisted simply because there's already too many students in the class who have a similar background/personality type as you do, and there's nothing you can do to control that, you can't control the other applicants and interviewees--the most you can do is prepare as best as you can, be yourself, and hope for the best.