Elise Proulx has written an excellent article about the science and art of finding an agent. Check it out.
As a literary scout and former literary agent, I've read at least a gazillion pitches. While the quality that makes a book perfectly sellable may be elusive, there are ways to turn on and, conversely, turn off an agent before he or she has even read your magnum opus. Here's some of what I've learned about the right way to make that first connection with an agent.
The first step in finding an agent is networking. Go to literary conferences that feature agents, try to hit literary events—readings, book parties, and so forth—join a writer's workshop, ask around. It's always preferable to have a personal connection to an agent. Your work will get more attention and a quicker response.