I like this 'fill in the blanks' guide to writing a premise for a film - it works just as well for books. (Thanks Nelson in Toronto!)
It's helpful in making you concentrate on the main points and, maybe more importantly, the main selling points! Now I'm not being mercenary here, after all, once you've written a book you've got to sell it to a publisher before anyone else is going to get the chance to see it. Even if it goes against the grain, its only fair to posterity to make that effort.