Research appropriate publishers. It is a waste of time to send a proposal for a non-fiction book to a publishing house that only deals with fiction. Get as much detail as you can about your targeted publishers. If necessary, ring and find out the name of the appropriate commissioning editor. You need to know the exact requirements of the publisher; for instance, some will prefer e-mail submissions while others will want you to send a query letter. Follow all the given instructions.
Begin your proposal with a precise summary of your book. You may decide to give a small amount of information on what inspired the idea. Provide a chapter breakdown. Include a brief autobiographical section. The most important thing here is to explain briefly what makes you the best person to write this book. This is not the time to be modest about any relevant achievements or qualifications.
Describe where you see your book fitting into the current market. Mention competing books, but be careful how you do this. You want the publisher to know that you have carried out your research, but you do not want to give the impression that you are entering an already crowded field. Describe the intended audience. Mention any marketing expertise, you have, or any contacts who would help promote your book.