Yury, from Pythian and the Sydney Oracle Meetup, has mentioned Toastmasters. Given my audience is mostly developers, let me sum it up my way: Toastmasters is a test environment for public speaking.
As with a test environment, you're expected to have a few "does not meet requirements" incidents. That's why it is there. You can experience those, and learn from them, and repeat with improvements. And none of that damages "production". It is a dedicated, safe environment for rehearsals.
I can't speak for all groups, but in the couple of meetings I have attended, as a guest, I've seen positive and constructive criticism. As well as learning to speak, others are evaluating...which means listening. The talks are only a few minutes, so I don't bore people with techy details. There's no sign of Powerpoint either, so it isn't quite a conference rehearsal.
Again, like testing, this means you are using 'test' data, rather than volume testing. You can focus on the fundamentals of communicating rather than on content.
I plan to sign up fully in the near future, so there might be more coming in this blog, or it might come through in my Google Plus posts (apparently I am in the circles of over a thousand people, but I'm none the wiser on how many actually read anything I post).