I had a big interview last year for dream job. I felt under enormous pressure as I knew in my industry this opportunity would be years before it came up again so it was then or never really.
What helped me was scrutinising the job spec as they had issued it and considering how I would answer all part of it with an relevant 'story' to back up my experience.
I also prepared stock answers such as 'tell us about yourself' so I knew exactly what information I wanted to get across in this way
I also really researched the role and felt confident that I was knowledgable about it
Basically what I am saying is I sat with a friend, in fact I got advice from a couple of people and built a mental map of what they might ask and how I might answer, always with an illustrative example from experience.
Finally, make sure you are on time / early so not flustered. Have a bottle of water with you. Take your time getting settled on the chair, sit comfortably no fidgeting, make a conscious effort to relax your body so your shoulders are not hunched, practice radiating 'open-ness and calm no matter how you feel inside. Practice in front of your mirror!
Take deep breaths and speak slowly. Don't be afraid to say 'could I have a moment to gather my thoughts if you are thrown something left field.
It worked - I got the job!
Good luck - you can do it!!