Yes, try to look pleased with yourself even if it goes wrong!
I'm an ex-nervous wreck so this subject is close to my heart.
Remember that the examiner is on your side and wants you to show what you can do, they are willing you to do well. It's not an audition, you are not competing with anyone else.
Play in front of others so that you experience how nerves will affect you. They will affect you, so don't kid yourself you will be able to not be nervous. Learn how you respond to nerves so there is no surprise on the day about how you will feel, and so you will know what feelings to expect and how it will affect your playing. Practice in your mind what you will say to yourself and how you will react when (not if) you make a mistake. If you prepare how to cope with mistakes in this way, hopefully one mistake will not make the rest of your performance crumble. Also, practice practice practice practice practice.....reach for a level beyond what you are being examined in so it feels easy. Then hopefully when you factor in nerves it will still be comfortable enough to play in spite of them.