Do real mock interviews with people who will actually push back on weak answers: your pre-health advising office, a current medical student, or a friend who won't just tell you it went great. Practice your core stories out loud repeatedly until they sound natural in conversation rather than memorized and recited.
Research each school's specific mission, curriculum structure, and any distinctive programs so your "why this school" answer is concrete rather than something that could apply to any program. Prepare two or three genuinely thoughtful questions to ask your interviewers. And be consistently kind to everyone you interact with that day, including administrative staff — they talk to the committee, and it's noticed more often than applicants expect.