My advice--or rather, the advice of Dean Wesley Smith and Kristine Kathryn Rusch--is this: don't bother with an agent. Publishers use them as excuses when they don't want to buy a submission, and there are a lot of comments, especially at DWS's blog, with agent horror stories. (Actually, I think there are some in one of the recent editions of KKR's Business Rusch.) Plus, with so many agents turning to digital publishing, there's not much of a guarantee that an agent will send your manuscript to as many editors as possible before deciding it's unsaleable and offering to publish it themselves, for a big cut, of course.

Naturally, you don't have to listen to me, but I encourage you to read those blogs and form your own opinions! They're valuable in many ways besides the agent issue.