I am still working on my public speaking skills / technique. I have had some really great talks this year but also some that I'm not so proud of.
If I had to pinpoint an 80/20 rule here, it would be around these two things: practice and experience, in that order.
To prepare, I usually like to have either a really strong idea for a talk, or a really well received blog post, OSS project, or video to build upon.
The more experience I have, the more I'm loving the blog post to conference pipeline. Finding an idea, then deep diving / exploring it as you write a blog post about it allows you to really learn something beyond the surface.
From there, writing a talk becomes really easy as you've already solidified and crystalized your thoughts around the topic, and even have talking points to work with.
I still get nervous sometimes, but only in rare situations (like a big keynote or something), but the anxiety has subsided quite a bit from where I was 5 years ago (but not completely gone 😅).