You can do deep breathing exercises before the test or when you have a few minutes , you can walk across the examination room. For TOEFL test, remember to use good vocabulary, speak clearly and enunciate words well.
Do not forget to use good structure in making your sentences. Also, it will help a lot if you follow the time limit. Just relax , be confident and do your best. If you prepared well prior to the test then you will do well, no need to worry.