Start with a good idea. While a good idea won't necessarily sell itself, it's essential to securing a book contract. For nonfiction, publishers are looking for a topic that hasn't been covered by dozens of other writers. For fiction, try a new approach to a time-tested formula.
Consider hiring an agent. Many publishing houses will not talk to writers who aren't represented by a literary agent, especially those dealing with fiction. An agent generally takes from 5 to 15 percent of your earnings, but a good agent has contacts that can create opportunities for you. He can also do much of the legwork involved in selling a book. Look for agents in the current "Writer's Market" at the library or get recommendations from writers forums and websites.
Write a proposal. To get a nonfiction book considered, you'll have to write a formal proposal with an outline of your idea, a sample chapter, a little about your background and experience and how you intend to promote your book. For fiction works, generally a few chapters accompanied by a cover letter will suffice.
Have realistic expectations. Few writers get rich with their first book contract. Unless you are a celebrity or sports star, expect your advance to be in four figures, not six.
Be patient and persistent. Like any other sales venture, selling a book is a numbers game. That is, the more people you talk to, the greater the chance that you'll make a sale. Set aside your ego and your personal involvement as much as you can when contacting publishing houses.