Visit your local library or bookseller to see what books similar to yours have recently been released. Let's say you've written a book on fitness and exercise. Look at all the newer books on fitness and exercise, and turn immediately to the acknowledgments pages. Authors almost always name their agents in their acknowledgments. The same would apply to a fiction book. If, for example, you've written a cozy mystery, seek out all the latest books in that genre. A quick check of the acknowledgment pages will give you names of representatives. Keep in mind that just because top agent Mr. Jay represents the best-selling author whose book you're holding is no guarantee he will agree to represent you. However, a junior agent in his firm might agree to take you on.
Visit the Association of Authors' Representatives (URL below). If you're new to the book publishing business, you might want to read through their frequently asked questions. Then move on to the search-able database. Taking a cue from the two book scenarios above, if you search for "exercise" or "fitness" and fail to get a hit, try "health." This will bring up a number of agents who are members of the Association of Authors' Representatives (AAR). If you search for "mystery," you will get agents who are actively seeking mystery authors. Not all book agents must be members of AAR, but you can be sure members of AAR are reputable.
Take the list you've compiled via your local library and/or bookseller and AAR, and do a search for the agents on your list. (Most agents have websites and AAR pages usually have agents' URLs listed.) The agent's site will provide you with a list of his clients, a list of books he's recently represented successfully, all the agents within the firm and submission guidelines. If permitted, you may query by email. This will provide a speedier response. Be aware, however, that an email query must be as professional and carefully crafted as a snail mail letter.
Make your query letter concise (no more than two pages). Briefly tell the agent what your book is about, but avoid grandiose comparisons. Statements such as "I'm the next Stephen King" or "everybody tells me this book is the best they've ever read" immediately brand you as an amateur. If the book is great and if you're a terrific author, the manuscript will speak for itself.
Prepare your book description similar to the copy you would read on the back of a book. One example, "Rachel Hollins buys a ring from a flea market and gets more than she bargains for. Turns out the ring is a 19th century mourning ring which once belonged to Claire Marcette, a woman murdered in 1920. When Rachel attempts to return the ring to one of Claire's family members, she gets involved in a modern-day murder rooted in the past." In the case of a nonfiction book (the exercise book in our nonfiction scenario above), your description might read something like this: "Just 10 minutes a day, four days a week, are all you need to have the body of your dreams. I can show you how to boost your metabolism in those 10 minutes to create an on-going cycle to help you lose weight and feel great."
Give the agent any relative experience you have. "My short stories have appeared in Modern Fiction, Women's Fiction and Fiction Today." If you're the nonfiction exercise expert, you might say, "I have a Ph.D. in Physical Education and I developed the methods utilized in my exercise book during a six-week trial of patients enrolled in the Weight Loss Research Center."
Write your synopsis if the agent's submission guidelines request one. To prepare a synopsis, give a brief account (again, no more than two pages unless otherwise specified in the guidelines) of your book's plot. For the nonfiction book, provide the table of contents and a sentence or two describing each chapter.
Wait for an answer. If you're rejected by the first agent you pitch to, move on to the next one on your list. Good luck!