So you're on the hunt for an agent? Of course, referrals are always the best and fastest way to get an agent to look at your work - so this should always be your first step. Ask your friends, your writing teacher, your mailman, or even your local barista about their agents and see if they'll put in a good word for you. I'm telling you, you get vouched for like Donnie Brasco and it's an immediate bump to the front of the list.
For most people though this isn't an option, so the next step is some general research, and thankfully for writers in recent years this has become much easier. The web is of course the easiest medium - Agent Query, Writers Net, and Publisher's Marketplace are all great websites. Most agents have their own websites too, but just remember that plenty of fantastic ones don't at this point (Curtis Brown, Harold Ober, Susan Golomb). I also suggest going to the library and looking at Literary Marketplace (its $300, so unless you're rolling in it perhaps its better not to buy), or going to the bookstore and picking up Jeff Herman's Guide. While you're there here are some more books to check out if you're serious about this whole "getting published" business.
Michael Korda, ANOTHER LIFE. Now semi-retired, Korda has long been one of the superstar editors in the business. An insider's view that reads like fiction.
Mark Levine, NEGOTIATING A BOOK CONTRACT. I'm telling you, this is publishing's dirty little secret, like that night I.... In this lucid little book Levine addresses each section of a book contract, concerns an author/agent should have, and alternative terms you can ask for. If your agent doesn't have a legal background, an attorney on staff or on retainer, or at least some previous experience with contracts, than they better have this on their shelves. Levine is releasing a new edition in May.
Jonathan Kirsch, KIRSCH'S HANDBOOK OF PUBLISHING LAW. You need to know your rights, and Kirsch spells them out for you.
Noah Lukeman, THE FIRST FIVE PAGES. Full disclosure - I interned for Noah years ago when I was first starting out. Still, I can confidently recommend this without a hint of bias. This handy book gives suggestions on how to avoid all the pitfalls that will cause your manuscript to get immediately rejected.
So now you have a huge list of agents and a pile of books. The next thing is to get the list down to a manageable size. First, make sure to check out SCFW's website about bad agents, as well as the Predators and Editors website. Anyone who charges a fee should immediately be bumped off your list. Anyone who charges more than fifteen percent for domestic sales has to go, as does anyone charging more than twenty-five percent for foreign sales. Anyone who is either not a member of the Association of Author Representatives or doesn't agree to abide by their Canon of Ethics gets the boot too.
You still have a huge list of agents, but we're getting there. I'm a big believer in querying early and often, but with some discretion. I suggest separating the agents left into at least three hierarchical groups. How do you break this down? Well, did that agent represent a book/author you really like? Did you see them at a conference and you dug what they had to say? Do you want an agent who lives in New York? This last one certainly isn't essential (see Peter Ginsburg, Jim Hornfischer, Sandy Dijkstra, Nina Graybill, Elaine English, Kristin Nelson, Ted Weinstein... and many more), but some authors really want an NYC agent.
So lets say now you have an "A" list of agents. Once you have your query together (and that's a completely different topic) you can start making your pitches in accordance with each agency's guidelines. There's no need to send it as an exclusive unless the agent requires it, and if they do than maybe you don't want to try them anyway.