Don't forget a blanket or warm clothes. Socks are especially helpful. Close your eyes. Completely relax your mouth and your eyes. This is actually a technique that I use in training singers.
Relaxation begins with the eyes and the mouth.
If you completely relax those two features in your face the rest of your body will follow suit. Granted there may still be some tension but we will take care of the rest. Keep this technique in mind. If you lose focus always return to this step and begin again. This is the most important step.