This is a bit American, but it helped me once:-
To have the most pleasant interview experience possible, it is important for the interviewee to be calm and comfortable throughout the process. Being edgy, overly talkative, visibly panicky or uncomfortable, and fidgety are surefire red flags during an interview, so focus on settling down beforehand to make it go as smoothly as possible for both parties involved.
Relaxing During the Interview
* Take deep breaths. Breathing a full deep breath through your nose before answering each question will not only calm you by sending oxygen to your brain, but it will also give you time to think about and organize your answer. By collecting your thoughts before speaking, you will feel more confident about your answer and will be less likely to ramble.
* Maintain an appropriate but comfortable posture. Don't slouch, but don't be overly rigid. If posture is normally a problem, practice a comfortable and poised position that you can maintain without pain or feeling self-conscious. If you worry about your own posture during interviews, mimic the posture of the interviewer (when appropriate) – his or her posture should communicate what is considered acceptable.
* Feel free to be yourself. If you normally crack jokes, don't feel that you have to leave your sense of humor at home. You are not there to convince the interviewer that you are someone you are not (hopefully!), so be genuine and let your personality shine through. You will feel much more comfortable if you can focus on your questions and answers instead of whether you seem too shy or too bubbly.
* Know how you express your nervousness. If you think about how you have acted in the past while nervous, chances are you can find a pattern in your behaviour. Do you run your mouth? Do you forget what you were going to say? Do you bounce your knee? Do you sweat profusely? Then prepare to address the problem. Think about your answer before you say it, keep a hand on your knee to realize more quickly when it starts to bounce, take a dress handkerchief with you in your pocket, etc.
* Remember: you are there because they want you to be! Keep that in mind when you get nervous during the interview – they were already impressed enough to invite you in for an in-depth discussion. That means you have something they want, and feeling that you have something valuable to offer them can put you at ease – the power doesn't rest solely on the other side of the interview table. So think positively.
These ARE clichés but are important too:- Smile and be yourself.
Good luck with it.