Try mock interviewing or videotaping yourself. I didn't do this but a lot of people I know did and swore it helped them.
The main problem people have when interviewing is thinking that they have to answer the question the second it is out of the interviewers mouth. Take a breath to clear your head a bit, and focus on calmly and slowly answering. It's not a race to get as much information out as quickly as possible.
It really helped me to jot down some points to common interview questions (weaknesses, strengths, why I want to be a doctor, why XYZ school, etc.) in advance. Be careful not to make your answer sound memorized though.
It also really helped me to let myself freak out about an hour before the interview so I had time to recollect my thoughts and calm down. I found that, if I tried to hold in my nerves completely, I would get jittery right before the interview. I felt bad at my interviews for a few of the other interviewees who were fine until 2 minutes before they had to go to interview. That is not the time to let yourself get worked up.
As someone said above, try to just be yourself. My best interviews were ones where I actually just let the conversation flow and actually got choked up talking about events in my life (one interview discussion ended up leading to a talk about my mother and sister nearly dying) or things I'm really passionate about. Just be in the moment, and let the interviewer see who you really are rather than just a compilation of your stats and activities.