When I was reviewing material on writing a fiction proposal, I found that the advice out there is rather disparate in some regards. The elements of the proposal are often similar, but the style and method are usually not.
Basically, there is no one particular version of a proposal that will necessarily make your book succeed or fail (as long as you include all the required material in a logical, consistent manner). Some agencies or publishers may require more or less than others, but here are the main elements you definitely need to include.
The information here is tailored for the fiction market. However, you can definitely modify it for other types of proposals (non-fiction, short fiction, magazine articles, etc.).
Cover Page: Actually, you’ll likely be emailing proposals, which means you’ll need a great letter to introduce you and the work, but make sure that you include a title (and possibly a short tagline) and your contact info.
The Premise: This is not a book report; this is where you tell the person you want to buy into your work why they should. Some suggest 250 words, and others say 500. If you can describe it in 250 words, then do so. But don’t worry if it takes more. Spend as much time as you need working on this, as you will likely sell your work—or not—here.
Who Wants It: Ready to be a sociologist? Well, in some ways you’ll need to be because part of the proposal process is telling who would want to read your work. Hopefully, you know the market you’re writing for because you yourself have been reading in that market. But take some time looking through a bookstore at the books in a particular genre or field. Look at what the publishers put on the front and back, how they package the book (colors, pictures), and what types of books are near each other.
Detailed Summary: Hopefully you already have a good grip on what will be in your book by this point. Some say that you should be finished before you write a proposal, while others suggest not finishing. Me? I wanted to be finished with most of it first so that I could have something tangible. In any case, once you’ve sold your work in 500 words or less, this is the part where you describe the sections and/or chapters inside. Some agents and publishers prefer an outline; some prefer a chapter synopsis; and some want both. Either way, expect to spend time here. Also, make sure you tell the status of the manuscript. Is it finished? Have you even started?
Who You Are: Once you’ve pitched your idea, it’s time to pitch yourself. Explain why you’re qualified to write what you’ve proposed, what experience you have, and a list of other works you’ve done. Also, you may want to include how you’ll be active in promoting the work on the Internet and other means.
As I said, I have yet to find one set method of proposal writing out there, but all of them contained these basic elements. Just make sure to keep it structured and logical.