Back cover copy

Dzur is out in paperback, and has one of the best back cover write-ups I've ever gotten, and I've gotten some good ones.  How do people do this?  I write the books, but every effort of mine to summarize them has just made them sound stupid.  I finally stopped doing that, because I realized if I kept it up, I'd think they were stupid and wouldn't be able to write any more.

I have no doubt that there is an "art" to this sort of thing; but is there also a craft?  What are some of the techniques people use to summarize books so as to not make them sound stupid (and why can't more reviewers learn them)?