How do you convince an overly confident, highly senior executive that it makes sense to rehearse tomorrow's presentation? “Me, hey, I have given thousands of presentations in my career, I will wing this one!”
Winging does not work, you need to know your substance in-and-out in order to be spontaneous. A presentation without rehearsing produces of one of two possible results: a poor performance, or a replay of the presentation you gave last time without emphasizing the new content in the deck.
I do not have the ultimate answer. You can say that Steve Jobs took days to rehearse a keynote. You can scare her and say that the analysts in the audience tomorrow see hundreds of presenters each year, and you better stand out. Or sometimes, people agree to go through the presentation with an outsider like me, the presentation designer, in a closed room where non of their subordinates have to hear the corrections we discuss.
What does work for you?