Just have questions ready and relax, that's it really. More often than not, people interviewing you is very impressed with themselves due to the position of power they hold over you and subsequently just LOVE to here themselves speak. Every interview I've ever had or witnessed this was the case. Keep them talking by asking pointed questions about the company and let them share their wisdom with you. They'll love you for that and you'll probably end up walking away with the job.
Make it a conversation and not a Q & A.
While I agree with the 'conversation over Q & A' bit, I can't agree with the first bit since I've had to interview people myself in the past

It might be a bit late now, but if you got the interview through an employment agency then ask them what will be expected of you in the interview. The agency will have talked with the employer about what type of person they are looking for and the tone of the interview and should be able to give you some pretty useful information.
Also remember that this isn't a one-way thing. The company wants to employ someone and there isn't any reason why it shouldn't be you. They have invited you along because you have on paper the skills they need.
For my current job, one of my questions was 'What do you like best about working here?'. I think it knocked by boss back a bit at the time, but I think it shows that you care that the job, company and work-style is going to suit you, which is very important.
Apart from that... be polite
