What makes a good book trailer?

I'm presenting at a panel for the upcoming Continuum 8 SF convention, and the title of my panel is 'Book Trailers: what makes a good one, what doesn't work and why.'

Now I have many opinions on the subject, but I'd really love to hear from everyone else before I try to make a list of the do's and dont's. So please, leave comments below about your ideas, or better yet, link to your favourite (or least favourite) examples.

Thanks,

TB

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