Blue:
I've worked in Broadcasting for 18 Years. I've done voice over work, and been on air talent, I was a Pro Wrestler & Public Speaking is what we'd call "Cutting a Promo"
I never took a lesson in it, But I'm told I'm fairly good. You just believe in what your saying or selling, and bring it. I know it sounds ridiculously simple, but it just is man.
Pick a topic & Study it, then go to the mirror & practice.
If you want a example, I've been trying to raise funds to get the Film "Nation of Heroes" shot. Here I am at the Sunscreen festival Pitching it. Many industry Pro's have told me this was the best Pitch they had seen in their lives. (I'm actually negotiating about getting the film makers their funds now) Take a look.