Public speaking is, for many, one of the most challenging things to do. Long story short, I used to be terrified of public speaking and now I love it.
How did I overcome it? First, I had to do it so I was forced to practice. Second, I attended many Toastmasters evenings and worked on my presentation.
Today I still get the chance to do public speaking several times a month and I am very relaxed. Every now and then a moment of fear arises but it quickly passes by the time I actually start speaking.
Knowing your subject is a big advantage. Since you've been doing GTD for a number of years you will be well prepared on the topic. I remember with Toastmasters one of the practices was to get up and talk on any subject that was given to you. It was fun to try and be 'on' when you are just winging it.
Being in sales you will find what you learn from public speaking will only benefit you in your professional life.
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Mastering Workflow (2012 & 2013)