I think for a lot of schools (but not all) it can give you a leg up to interview early decision. Schools know they can fill their spots with people they like, without playing the wait and see game that happens sometimes right up until school is about to start.
As far as the interview goes, "be yourself - but better" meaning highlight all your strengths and have your deficiencies minimized. Practice, practice, practice as many potential questions as you can find. Answer them succinctly with real examples that support your answer and application.
As for what not to do, here is a cute video that I'm sure interviewers will laugh at (because you've experienced it) and interviewees will cringe at (because you fear it)!