Easier to look a fool telling jokes than look a fool making a valid point.
edit: or Toastmasters
Among other things, Toastmasters will cure you of what ails many accomplished speakers. That is, the verbal or non-verbal tic (umm, ahh, so, like, wild gestures, hands in pocket).
And you would be amazed at the kinds of mistakes that you can make!
A few years back I did a 3 hour tutorial at OSCON. I put a lot of effort into making sure that I had 3 hours of material I planned on presenting, with some extra I could slip in. I "presented" to an empty room and was sure of my timing.
Turns out that I speak quite a bit faster in front of a real audience. I was done in 2.5 hours, including what I planned on leaving out...
Then again, it's probably doubly cringeworthy if I did see it. Once for the mistakes I would see, and once again for the difference in how well I felt I did to what I imagine would be apparent once I could review it.
And also it lets you make a fool of yourself with a smaller audience.
Very few people are good at something the first time they try it. Why do you imagine you're a great presenter the first time out? As with anything else, make your mistakes in small domains.