Hmm, maybe this is actually a problem the community can address. For our conferences, people who are new to giving talks normally have a practice run or two at their universities with local people. Then they get feedback how to improve the talk. It doesn't work wonders, but it does help quite a bit. Not only does the feedback help improve the slides, the style of presentation, it also builds confidence.

Maybe CCC/somebody could organize optional "training sessions" where people can give test-runs. I'd volunteer to listen and give feedback. Or maybe a tutorial: "How to give a good congress talk"