If you know your subject and are prepared, turn the adrenaline rush of stage fright into power. What you are feeling is an excess of adrenaline that gives you the power of “fight or flight”. Take a few minutes before going on stage and do this exercise -
Stand tall with your feet shoulder width apart
Imagine moving the adrenaline from the top of your head down your body into the floor under your feet
Visualize that adrenaline as roots under your feet
Now move just the power part of the adrenaline back up into your body
Take a second to enjoy that feeling - throw your shoulders back - feel big
Step away from the spot you were in, leaving those roots of excess adrenaline behind
I have used this exercise many times before performing and have found it to be very helpful.