I started off doing pretty badly on my interviews and learned a lot on each one. I was deferred at one school, waitlisted at 2, then finally accepted at my #1 choice - so while I don't have mad acceptances, I do think I figured out how to do better on my interviews.
1. Be relaxed. My first interview, I was so nervous that I really just didn't stand a chance. You just have to do whatever it takes to make yourself relaxed - don't bother with figuring out the public transit system the morning of the interview, just take the darned cab. Don't take a flight home within 2 hours of the end of the interview day unless it'll save you more than $100. The extra money is worth being more composed during your interview.
2. Make sure your interview tells a story. If you have a great personality, fantastic - that's all you need. If, like me, you're somebody who is subdued and easygoing, recognize that you may come across as boring, and fix that by emphasizing (completely true and honest) specifics.
For example: In response to the question "So, what was it like, running a half-marathon?"
Boring answer: "Oh, it was really cool. Training was really hard, but it was a great feeling in the end." (Yawn. This tells you nothing. Of course it was hard, and of course the end feeling was great.)
Better answer: "It was the best thing I did that summer. Seeing my family rooting me on at the finish line - especially my half-sister, I hadn't seen her in years - meant more to me than I expected. I'm training for a full marathon right now.."
Now the interviewer can ask you about the full marathon OR why you hadn't seen your half-sister (And of course, you wouldn't have mentioned your half-sister if the true answer wasn't positive, like "we hate each other." Instead, you'd be mentioning her because the answer is "She was in the air force - most of my family is in the military, actually" or "I was in the peace corps, actually." Something that tells them another positive thing about you.)
Again, these should be completely true stories, just making this up for an example. If you think hard enough, you can relate most potentially boring questions to a story that tells the interviewer something good about yourself. It's good to think up possible examples in advance.
3. Re-emphasize the best points of your application. I had a couple of great volunteering experiences that were the main theme of my PS, mentioned again in my LORs, and included in my Activities - so I started off not bothering to mention them in my interview. But once I started doing it (in response to questions where it made sense), the interviewer's face would light up and they'd immediately shift the topic to those experiences. This was almost always the strongest part of my interview.
Don't count on the interviewer selling you to the committee based on your strong points when you haven't bothered to emphasize them to your interviewer first. I was avoiding it because I didn't want to come across as conceited - but there's a huge difference between selling yourself and being conceited, and you have to sell yourself a little.