I just had a big interview for a job I wanted. Things that will help:

1. Go with all the confidence you can muster. Don't let yourself be shy or self-effacing. That should not be hard for someone who is used to meeting strangers.  

2. Think about all the questions they could ask. Then prepare answers for those questions. I have a "how do you work with difficult people" answer, I have a "what is your biggest weakness" answer, etc. Polish those stories so they're tight and hit the main points.  

3. A good interviewer will look for competency, commitment, and chemistry. Think about all three when you're being interviewed. A suit-and-tie shop is going to be different than a tech startup. Where will you best fit in? You'll know a bit when you show up, but do some research now to see what they're culture is like. And then, if you're a good fit, make sure to indicate that during the interview.  

Good luck. I do a lot of hiring, and I'm always more excited to see someone who wants the job I have rather than just wants a job.