I wanted an agent who would actually sell stuff. After two British agents failed comprehensively, I was reading Locus (the SF field's trade journal) and noticed a press release about an experienced editor leaving her job to join an agent in setting up a new agency. And I went "aha!" - because what you need is an agent who knows the industry but who doesn't have a huge list of famous clients whose needs will inevitably be put ahead of you. So I emailed her, and ... well, 11 years later I am the client listed at the top of her masthead!