Deep breathing. And if you're nervous - say so! There's no point in trying to hide it - one, they know it anyway, or at least expect and understand it - two - they can probably see right through any forced confidence....
Remember also, that they are as much in the dark as you are.... they are seeking to fill a post based on a CV and a short time spent with you....
Believe in yourself. Know that you are up to standard, and be enthusiastic,but not too eager.
Practise at home, with a good friend.
watch for questions that ask 'what would you do in such-and-such a situation?'
Don't be afraid to pause and think, before answering. Interviewers prefer somebody who takes time to consider a question, rather than someone who hastily jumps in with a pat or prepared response.... they may think you're saying what they want to hear, not what you really believe....
Dress code?
Always look the part.
If necessary, ring ahead and ask a few questions to the receptionist, about the company, and the dress-code. Show an interest, and make a friend. People talk, you know, and a good word, ahead of the interview, may keep you ahead of the game....
Hope it helps!
