As someone who went from 'nervous the entire day leading up to The Talk' to knocking a presentation out of the park with not much more prep than bullet points, I agree with goobster in that there is no trick out there. There's practice, in the case of public speaking practice really makes perfect.

I do recognize the flipped-switch feeling. For him it was improv that did it, for me it was a debating course. What I think is key to getting comfortable is having some kind of structure that really works for you. Essentially, you're telling a story, and there are many tools that can help you do that. Find the tools that fit you.

For me the most useful tools to enhance my speaking abilities were debating methods like "tell, tell, tell", articulation / manner guidelines. I realized that when I have my presentation's core arguments / structure nailed down, I can just explain / improvize what I want to tell around it. I'm guessing goobster found a bunch of useful tools in his improv classes.

Genuine feedback and constructive criticism was also an eye-opener to me; other people are way better at pointing out your idiosyncrasies than you think.