Actually there's not much you can do to relax. I've heard about breathing exercises and such. But nothing worked for me. The good thing is most of the interviews are very laid back.
Once you start the interview you'll be nervous, but the weird thing is, you'll probably find yourself relaxing after a few minutes and going with the flow. Halfway through the interview you might be surprised to find yourself fairly relaxed, especially if the interview is going well. It's okay to be nervous, but also be confident in yourself and your application.
If there are weaknesses in your application, make sure you are ready to explain them if you're asked.
And if anything, relax afterwards. I've seen people getting interviewed at OSU, even after they were over, man they were would tight. I felt like telling 'em "you can breath, its over".
Good luck!