Amee's right. Some people naturally do better under pressure than others. Here's some hints that help me under pressure:
*Wear gloves to keep your hands warm
*Breathing technique- put your thumb over one nostril keeping the other open. Breath deep, and then, at the peak of your breath, put your pinky over the other nostril and exhale. I know it sounds stupid but it really works!
*Don't practice the same day of the performance. If it is ready, the piece will come naturally.
Hope these help!