I did some improv about ten years ago, when I was also doing some amateur theater, and I noticed that the next time I did a play, I felt almost zero nerves on opening night. If you can act in front of people with no script, all the scripted acting and speaking suddenly feels like child play.

Other advice would be to film yourself speaking and then watch to figure out where you're showing nervous tics or saying "you know," "um," etc. You can weed this filler out.

Know your material and be passionate about it, if possible. I know that some sorts of public speaking, such as work training, might be more a job than anything you love, and I'm not sure how to make that particularly engaging.