These kind of imaginary scenarios happen to most of us when faced with public speaking or performance. Whether it's a meeting, a presentation or even just playing charades with a group of people you don't know very well, anxieties about failing and looking silly can rise up to haunt even the most seemingly-confident among us. Take any opportunity you can to speak in front of others, even just the local PTO or a party. Imagine you're floating above your audience, orchestrating the event like a director. Treat it almost as a musical piece, focusing like a singer would on the breath and rhythm of your words.
Mild exercise helps burn off some of the jitters.
When we're frightened or nervous, our breathing becomes short and shallow.