Here are three simple tips that could make a difference to your score:
1. Be confident
Although you might be nervous, try to speak clearly and confidently. Smile and be friendly* with the examiner. As you go into the exam, tell yourself that you have prepared well and that you're going to enjoy the challenge.
2. Know that you are prepared
You will feel much more confident if you know that you have prepared and practised for the test. You should know exactly what to expect. For example, have you prepared some common topics (e.g. describe a place, a person, a hobby) for the short presentation? Are you ready for a past and a future question in part 3?
3. Speak naturally
Try to enjoy a normal conversation with the examiner. Instead of worrying about your grammar, listen carefully to the questions and try to give natural answers. Most of the questions are about you, your life and your opinions, so it's best to be open. I used to be an examiner, and I always found the job more interesting when students spoke openly about their opinions and experiences.
*Note: You are not marked on body language, but I'd still recommend that you try to act in a friendly, confident manner.