As a developmental editor specializing in fiction, YA fiction, narrative non-fiction and memoir, I find that once a client has decided to begin submitting his or her manuscript, many questions arise. First and foremost, "How do I get an agent?"
A query letter to a literary agent is still probably the most commonly used device to find representation. Unless you receive a personal referral or meet an agent at a conference or class, you will probably be writing a query letter. This letter is most likely the most important single page of words you will write in your career.
First, you must be able to succinctly describe your book, pointing out what is different about it, and dangling something in front of the agents' faces in order to make them request to read. A query letter is not a synopsis; think of it more as a "hook," and shorter is better than longer. Try to limit the letter to a single page, if at all possible.
Second, the writing must be stellar. This is an agent's first impression of your writing skills as well, so make sure that it's as well done as can be. "You only get one chance to make a first impression" is ever so true in terms of query letters. Think of creative approaches while also staying professional in presentation.
Entire books have been written about how to write a good query letter, so it's impossible to summarize all of that advice here. For my developmental editing clients, I offer help with the query letter and synopsis as part of my fee. Having someone else take a look at and advise you on a query letter to agents is a vital tool that authors should always consider.