Fear of public speaking and performance anxiety are very common things. Probably almost all of us have had the experience of desperately wishing to relax before a presentation.
This may be a situation where you should be careful of what you wish for. When I was younger I had a terrible fear of speaking in public. And my first job out of professional school required that I do many presentations before large audiences. I must have bought every book on the subjects of public speaking and relaxation training.
I alternated between the two topics. On the one hand, I figured that if I was really confident about my presentation skills I would feel more relaxed, on the other hand as the presentation grew closer I got more and more focused on how to relax.
Nothing really helped. In fact sometimes the talk seemed to go better when I focused on using the anxious energy.
One Fall day in Monterey California I find myself giving a presentation to a large group of doctors. For whatever reason, and I really have no idea why this happened, that morning I had almost no anxiety about the talk. No last minute reworking or second guessing the ideas, no desperate hunt in my mind for a funny intro. I was just calm.
The audience thought it was one of the worst talks I have ever given. They said I seemed flat, bored, and thoroughly uninterested in the presentation.
That set me to thinking about anxiety in a very different way. Anxiety, perhaps like depression, is not a mood to suppress or eliminate.
It is useful. A little bit of anxiety before a talk, at least for me, is necessary. The question became how to catch the wave and channel the anxious energy.
I had many images that helped me to do that better. One was imagining throwing the windsurfing sail up and into the wind.Another image that I often used, just before a talk, was to imagine that I was at the top of a roller coaster 
waiting to start a very exciting ride.
This helped me to claim or channel that energy into a more animated and vigorous talk.
I started thinking about anxiety in other situations in a different way as well.
I noticed that progressive muscle relaxation which involved first tensing muscles and then relaxing them seemed to be much more effective than trying to eliminate anxiety with breathing exercises or other exercises that focused entirely on the relaxation.