The symptoms of stage fright--a racing heart,excessive perspiration, hyperventilating, dry mouth and trembling--are triggered when too much adrenaline enters the bloodstream. Adrenaline is the chemical booster shot that pushes us into a mode of fight or flight when faced with anything that's perceived to be threatening.
Lack of concentration, sleep, or interest in the subject matter inhibit your ability to stay "on message," causing you to lose your train of thought.
Throughout childhood, parents teach their children not to draw attention to themselves; having to get up in front of others contradicts this instruction even into adulthood.
People crave acceptance in order to feel loved, respected and needed. When speakers worry about saying something their listeners won't like, it generates anxiety about rejection.
If you're self-conscious about your physical appearance compared to others, the anxiety will extend to insecurities about your material.
If you've ever drawn a blank during a speech, those awful memories will resurface every time you're in a similar situation. This fuels a self-fulfilling prophecy of failure even if you know your material inside out.
Websites like Owning the Stage and Anxiety Coach provide breathing and meditation exercises and advice about connecting with your audience in a positive way (see Resources).