Everyone has their own approach to starting a demo. Some start with a story, while others immediately start listing out the features of their products. Some hand out freebies right at the beginning to lighten the mood, while others try and crack a joke.
Yet, regardless of the approach taken to open your demo, the most important aspect is the result of the demo on the whole.
I've found one question to be most effective in bringing out the best in my demo's.