Sit down in a comfortable chair, close your eyes and relax. This is a good exercise to do at home the night before your big speech or presentation, but you can do it before you leave your office to go to the venue, or even sitting on the loo ten minutes before you have to get on your feet (you'll probably be there once the adrenalin starts to flow through your body anyway, so you may as well put the time to good use!).
Think of a time when you spoke in public but you felt you didn't do a good job, or the first time you ever felt nervous about speaking in public. See everything you saw then, and hear everything you heard. Don't see the image through your own eyes, but as if you were watching it on a TV or cinema screen (i.e. dissociated ). Don't get upset by it, just watch it unfold like a movie.
Now sit up straight and put a big grin on your face. Run the movie backwards, but at high speed like a Keystone Cops movie or the final scene in a Benny Hill show. Hear the people speaking backwards, as if you were rewinding a tape. Watch people actually swallow their words.
Now run it forward very fast, but change things. Give people silly voices. Give it a Benny Hill soundtrack. Make it into a cartoon and give people huge Mickey Mouse ears. If there is an individual in there who particularly upsets you, make him wear a tutu or some other ridiculous outfit. If he has a big nose, exaggerate it so it is like an elephant's trunk. If he has big ears, make them into Mickey Mouse's. Then run it backwards again. Then forwards. Then backwards, all the time at high speed. The first time you try this, do it at least a dozen times. Associate the Benny Hill music and the cartoon with the situation.