Preparation is the best antidote to nervousness. The better prepared you are, the less nervous you would feel.
The best method to score high in IELTS is to practice with actual IELTS exercises with a trainer. That way, you are answering the questions of the type they would ask you in the test and you are doing it with a qualified instructor who can spot your problems and weaknesses and help you get over them in quick time.
When you go in front of the examiner knowing fully well that you have done due preparation, you are bound to feel confident.