From time to time, I'll be discussing proposals or proposal packages and the aspiring author on the other end of the conversation will give me a blank look. So let's take a moment to talk about what goes into a book proposal, aside from blood, sweat, and wee bits of brain matter.
First off, though, you're generally going to have to open the conversation with a publishing professional via the use of a query letter (or e-mail nowadays, as likely as not).
Often, a positive response (or the option clause of a book contract) will ask you to send in a proposal. In the case of a novel, this consists of the manuscript's first three chapters or so. (I like to send between forty and sixty pages to give the agent/editor a representative chunk.) It should go without saying that these pages should be polished within an inch of their lives and gather momentum like an elephant on roller blades zipping down Mt. St. Lard. By all means, choose a stopping point that will (with any luck) leave the reader dying to spend more time with these characters and find out what happens to them. Never end mid-sentence (as I've seen done far too often in contest entries) but at the end of a scene or chapter.
Tragically, however, you're also going to need to include a synopsis. I say tragically since nearly everyone hates writing them and absolutely no one thinks their own are any good. (Okay, maybe some author, somewhere believes she rocks the synopsis, but I have never met her. And probably wouldn't like her all that much.)
The good news is, your synopsis doesn't have to be an absolutely brilliant work of art to get the job done. It does have to convey that you have some sense of what makes up a reasonably compelling character, can create a logical chain of events that's not too heavy on the cliches, understand reader expectations for the type of story you're telling, and have a handle on the basic plot points of any genre you might be writing. The sample chapters are the part where you really show off the freshness of your voice, clever twistiness of your writing, etc., etc. The synopsis is where you try to convince the reader you can be trusted not to muck up a perfectly brilliant beginning with a disastrous ending that needs tons of time-consuming and often-fruitless editorial work.
Things that can be helpful but aren't mandatory in a successful proposal:
1. A list of your "connections" with somebody famous-and-ready-to-endorse-your-novel (Oprah Winfrey, for example) or influential in the business
2. A Hollywood-like mashup comparison ("It's CATCHER IN THE RYE meets ANNE OF GREEN GABLES!") Unless it's honestly-reflective of the story and makes sense.
3. A one-or-two line high concept that makes everyone who reads it rabid to see more.
You get the idea. But just in case you're unclear, check out SlushPile Hell's list of things to leave out! (Be sure to click through the link to the uber-gruesome toad-skinned purse. Ha!)
One last disclaimer: This post discusses the novel proposal. Nonfiction proposals are a horse of a different color and often including a list of comparative titles, non-sequential sample chapters, and a chapter-by-chapter book outline rather than the novel's present tense, short story-like synopsis.