It's easy to become your own worst enemy when it comes to public speaking. There are a lot of myths floating around out there that don't serve any other purpose but to hold you back. Here are some examples from Dr. Gary Genard.
Check out his blog for more ideas to help you become a better speaker:
"Every actor, singer, dancer, musician, and other performing artist knows that a certain amount of pre-performance jitters is normal. More than that, it’s actually beneficial. It energizes you physically and mentally and gets you ready for the big game. But when you actually freeze when speaking, lose your focus, or suffer from anticipatory anxiety for days or weeks; when you avoid speaking situations or experience overwhelming physical symptoms, then something must be done."