Love this article from Dr. Nick Margan at HBR. Titled "Why You Must Rehearse," he tells the true story (details changed to protect the guilty) of a CEO who had a great story to tell and wanted to tell it to a large crowd and got that chance and then ...
The speaker began at the podium, but soon left it to roam the stage.
A couple of minutes in, he jumped up on the couch [on stage because the stage was so big] and executed what everyone figured out later must have been a half-remembered Kung Fu move. It was dramatic; the audience was riveted. Then he jumped down, uttered a few lines from the speech, and jumped up on the couch again, performing another semi-martial-arts maneuver, and a few more lines from the speech.
He kept up this astounding mixture of speaking and martial arts ballet until he had managed to get through — incoherently — about half the speech. Then he (mercifully) stopped and asked for questions.
There were none. 6,000 people in the audience were stunned into silence.
You see, the CEO/speaker who had this amazing story of success and apparently a "compelling" speech written for him didn't think he needed to rehearse.
Once everyone was happy with the speech, we proposed that the speaker rehearse. The CEO resisted, saying, "I'm very comfortable under pressure, because of my extensive martial arts training. I'll be fine."
So, what was the legacy left by this mangled presentation?
The CEO has never spoken in front of a large audience again. ... The organizer of the event has a 'bootleg' tape of the speech which is played at late night 'after event' parties to riotous laughter. They coined the phrase 'jumping the couch' from this incident to describe a speaker who melts down during a speech. [emphasis mine]
"Jumping the couch." I love it. I thought only Tom Cruise did stuff like that.
Don't let this happen to you. Don't ever believe that you can do your best without rehearsing, and then rehearsing again, and then again, and ... well, you get the picture.
Resource: The BEST book I've ever read on public speaking is Confessions of a Public Speaker by Scott Berkun. It's not the typical rah-rah, superficial stuff you get from many professional speakers. It's real, it's funny, and it contains great lessons. If you're serious about using public speaking as part of your marketing strategy, you should read it. It will make you better.