Well, first off relax ..heh easier said then done.
Okay, look at your application and your personal statement..know it well just in case you get grilled on it.
Look at the comments from previous applicants who were interviewed at your school, there's a link on SDN somewhere.
Practice interviewing with a friend or a parent.
and most importantly, the advice that my professor gave me
"Prepare for the worst" or rather expect the unexpected sothat when and if it does happen, you can handle it in a professional manner.
Good Luck!!!