What Maia said. There are agents and agents. Some, like Jeff Kleinman, everyone's heard of and when - as happened recently - he launched his own agency (
http://www.foliolit.com ), everyone sat up and took notice. there was even a small piece about him in the Wall St Journal. In the middle ground there are many well connected agents who will work just as hard on your behalf, but Jeff has a fearsome reputation because of his connections.
Lower down... if you're not connected, you can't be an agent. It's like the mafia, in a sense - without connections you're nothing. So unless you have a background in publishing, you'll likely never make it as an agent. If you're contacting someone new, find out who they are, where they came from, what they've done, and who else they're representing.
Alternatively - and the better route - set your targets high. Aim for a top of the tree agent, if you think your work is good enough. And if you don't, why are you looking for an agent at all?