Hiya, I'm in HR so ask me anything.
Little tips:
Breaking the ice is always a bit difficult. Start the conversation by telling them how easy it was finding their office or getting there by public transport, and how convenient the location is for you.
If they offer you a drink, always say yes. You might not want it now but you might get nervous during the interview and your mouth will get dry. Moreover, it's good to say yes!
Know the company, their turnover, number of employees, locations, etc.
Call your interviewer by his/her name, throughout the interview, and LOOK at him/her in the eyes when they ask you a question.
Try to be as honest as possible when they ask you a question about your CV. Employers will find out if you are telling fibs about previous jobs.
You should remember that they WANT YOU TO DO WELL during the interview, so be positive and stay yourself. They WANT to employ the right person and they want you to be right for the job.
What kind of job is it? Let me know and I can give you more tips.
Cheers
Mimixx