Here is a great article by Mike Evangelist in the Guardian that talks about what goes into one of Steve Jobs’ apparently effortless presentations:

With the demo set, my role was to stand by in case of technical problems with the software, or if Steve wanted to change anything. This gave me the opportunity to observe what was going on around me. The big keynotes require a very large crew with separate teams for each major task. One prepares the room to seat several thousand people. Another group builds the stage with its motorised pedestals, risers, trap doors, and so forth. A third manages the stage lighting, audio and effects.

Yet another sets up and calibrates the state-of-the-art projection systems (complete with redundant backup systems), and a huge remote video truck parked outside has its own crew handling video feeds for the webcasts and playback of any video needed during the show. Then there are the people who set up all the computers used in the keynote, each with at least one backup that can be instantly brought online with the flick of a switch.

It takes a few hundred people a couple of months to prep and deliver one of those presentations. The fact that Steve’s demos always just work is a big part of Apple’s brand. This is not by accident. (Via Presentation Zen.)