The best advice that I got given when I was applying for jobs is to research the company before going to the interview so you can recite some statistics to the interviewer (they always like to hear how great they are)
I didn't end up needing to do that but it wouldn't hurt your chances
Besides that, relax, I've had interviews where I should have done well but didn't because I got myself all worked up...
Good luck with your job hunt!