How much preparation for a demo?

It was interesting to read Greg Low’s notes on his TechEd keynote demo with Eric Rudder. It is easy to think people get on stage and ad lib through a demo they whipped up a few minutes prior. We all know this never happens and it is always the most prepared presentations that come across as the best. There must be some maxim along the lines of ‘the more preparation, the less it looks prepared’. I wonder how long Eric spent preparing?

Greg spent a lot of time preparing and the results were impressive – a slick (in the good sense), professional, informative demo. And it came across as a casual chat with audience.