There's no right way to write a proposal, and each will vary depending on the author, the author's agent, the subject matter, sample chapters, biography, and more. Still, I'll try over the next few weeks to provide a basic template to work from, starting today with the Overview (a typical proposal consists of an Overview, Marketing, Competitive Titles, Publishing Specifications, Author Bio and Sample Chapters).
The overview can be anywhere from a page to ten pages, though I've seen longer when no sample chapters are going to be available. Basically, you want to summarize your work and the entire proposal in a fluid and captivating manner. You can save some of the detail for the other sections, but I personally feel you need to briefly address marketing, biography, competitive titles, etc.
Besides getting a sense of the overall scope of the project, editors also want to get a sense of your writing skills. If this is a humor book, the proposal needs to be humorous. I know this seems self-evident, but you wouldn't believe how many times I've gotten a proposal or query about an allegedly funny project, and then when I read it it's like swallowing those grinds at the bottom of your coffee cup. In the same vein, if it's a history, it needs to merge multiple elements into a single, cognizant storyline that convinces me that this is a previously unexamined subject or that you're coming at it from a different angle. Writers should remember that often times this is all agents and editors will initially read; if you don't hook 'em here there's little chance they'll continue reading.
It's important to know what's going on behind the scenes when your agent submits your proposal to Editors X, Y, and Z... and yes, nowadays your agent should be submitting to multiple editors, unless you have an option or there's special circumstances that just make Editor Y the perfect person for it.
Let's say Editor X likes it. She or he will typically share the work with colleagues (often called back-up reads). Sometimes these colleagues simply don't have the time to read the whole proposal, and so again its essential that the overview kills. This point extends further. Let's say Editor X has got the support of the editorial staff. At some of the bigger houses the marketing people then will weigh in on it. Again, its the overview that is often looked at, and nothing else.
The overview is typically both the most important and the hardest part of the proposal to write. If you find yourself stuck on it, it makes sense to tackle the other sections (such as the outline) to get a better idea of the content you wish to present.