A pattern language on how to design your code to deal with demos is now online [1].  It also appears in the PloP2 book as chapter 25.

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An alternative view of demos (10 basic principles instead of patterns) is at http://www.coursey.com/html/great_demos.htm

Basically, it involves knowing your target audience, preparing really well, testing the presentation, revising, presenting and revise again.

While he didn't use patterns, I suspect David Coursey would feel right at home here:

"I’m sure most of my 10 steps seem more like common sense than magic."  -- DavidCoursey

KenMeltsner