The most important thing is to be very familiar with the company, and show a genuine interest in what they're doing. You should always bring a couple of copies of your resume just in case they turn up without a copy (I've known a lot of busy execs who did this) - it will show you're conscientious.
A lot of people I've interviewed tend to clam up and try not to say the wrong thing. If the employer needs to extrude information from you then it won't leave a good impression. It's your opportunity to shine in person after they've accepted you on paper. So be yourself, be vocal. If you don't know something, don't try to pretend you do, rather consider it something you're keen to learn about. You want to appear confident, but never over confident.
Also, it's a good idea to consider in advance some questions you have about the organisation. I always liked it when people asked about the company philosophy and future direction, since it implies they're considering the company as a long term proposition.
Good luck! :)