All these recommendations are good, but, ultimately it comes down to your qualifications. Interviewers (and I've done a couple, interesting sitting on the otherside..) can spot BS a mile away. If they've called you in, then you obviously have some qualifications. Important thing to do (and many times difficult) is to recognize these yourself and then be able to demonstrate them confidently. No amount of schmoozing will get you the job if you are an idiot (unless they themselves are idiots, but thats a diff story.)
Be genuine and self-aware. These people are looking for the right tool for a job. Prove that you are this, and present your best side, but DON"T exaggerate your qualities to make yourself look better. People are smart and you WILL get caught, and will only end up looking like an asshat, they will then wonder about ALL the things you've claimed, and you will be the first to be eliminated.
but in the end, don't be nervous. If you are right, you are right. If not, there will be another job that will be.
Good luck.