Determine what genre your book is--mystery, fiction, historical fiction, romance, etc.--then check online or in bookstores for agents who represent specialty areas. Make a list of the agents that best suit your book.
Write a proposal, which is a short overview of the book, and a letter of inquiry introducing yourself and asking the agent to consider representing the book. Send a sample of the manuscript--the first two chapters will do.
Make up 10 to 20 manila envelope packets with a self-addressed return postcard in each. Address each packet to a different agent. Include one of each of the items mentioned in Step 2.
Wait for any replies--emails or phone calls from agents, rejection letters or letters of inquiry written back to you. If an agent shows interest, discuss the project thoroughly before committing. Not all agents are created equal.
Keep in touch with the agent once an author-agent contract has been signed and while the agent is submitting the book to publishers. Promptly supply any additional information that is asked for.
Discuss the details of a contract offer with the agent when a publisher makes one.
Focus on working in a professional manner with the editor assigned to you once a contract is signed with a publisher. Making a book is a collaborative process. Compromise where you can and stand firm on what you feel is important for the completed book.
Work with an in-house publicist to promote the book before and after release. The publicist has contacts and will share some of those with you, so do follow-ups with her.
Create local events to publicize the book. Get on radio talk shows, write online press releases, blog and use social networking websites.
Plan author events. The publisher will already have a distribution network that will work for you to get the book to stores for author signings.