The Guide to Literary Agents has all the information you need. First, make a list of your criteria. By now you should know how to describe your own work, your genre and where you fit into it. Do you write fiction or nonfiction? Mystery or romance, thriller or fantasy, contemporary or historical? Are you a new writer who may need editorial help from time to time, or an MFA who only needs a contract negotiator? Second, comb through the Guide and make a list of agents who fit your criteria. Agents will tell you, if you read their blurb in the Guide, exactly what they do, what sort of writing they represent, and how they want you to approach them. (A word of caution here: if an agent wants three hundred dollars up front to read your manuscript, donít fall for it. Please donít feed the sharks.)
If your style resembles the style of a known writer then say that, but only if itís true. If itís not true an agent will know it, and then it counts against you. And donít tell an agent in your cover letter that youíve written the next Left Behind or Purpose Driven Life. Thatís what they all say. You want to stand out, not stand in line.
Follow directions. If an agentís blurb says ďNo phone queries,Ē then a phone call is the surest path to rejection. ďQuery onlyĒ means a letter onlyó one or two pages in a business envelope. Fifty pages means fifty pages. If Chapter 3 ends on page 52 then, okay, send 52ó but not six hundred. And donít forget the SASE. If an agent goes to the trouble of listing specific instructions in several different directories and a website sheís not going to say yes to someone who doesnít bother to read them.
Itís pretty simple, really. The way to an agentís heart is through a good book, presented with accuracy, clarity, and professionalism.