Jon Udell writes about how he prepares for giving a talk:
My process used to be…composing slides but now its turned into something completely different and quite surprising to me. As I discussed here, Ive finally trained myself to use dictation effectively. Ill go out for a long walk, like two or three hours, and dictate a rough draft of the talk. Im not able to do that continuously, I have to stop and think and start again, but I turn the recorder off during the think time so when Im done Ive got something approximating what the talk will be. Then I go for another long walk and listen to what I recorded, making notes about what slides to use. For last weeks talk I didnt take those notes in audio form, I scribbled them down while walking, but next time Im going to go back to audio capture. If you distill the long narrative down to short titles or phrases, its quick and easy to listen to a spoken distillation and write down the titles which become the armature for the slides.
The obvious reason why this works is that speaking out loud is good practice for speaking out loud. One of the subtler reasons is that exercise and fresh air really help. Another is that when Im away from my office and cant fiddle with a computer or look things up on the web, I have to literally think on my feet.