Interview is key... my program schedules at least two interviews with residents to get our input about the candidate.
I've interviewed a few this past match process. What I personally look for is someone who is easy to work with, not lazy, and asks questions about the program, which to me portrays an actual desire to be a psychiatrist, and not, "I'm doing this because everything else has longer hours."
Ask about where intern psych rotations are (which hospital, how is it like, is it at a general hospital setting or a pure psych facility, etc).
Ask questions pertaining to the child and geriatrics rotations/fellowship options.
There is a need for those specialties, and sometimes a candidate just vocalizing a curiosity about those fields (without the "ooh, i wanna be a geriatric psychiatrist!" comments) looks impressive, as if a student has actually thought about a future career and the process of their training.
Other than that, relax, be yourself, and have a good time. A pleasant conversation with the interviewers in addition to the other points mentioned above goes a long way when we fill out the evaluations afterwards.