Every writer starts off wanting (and usually needing) an agent and/or manager. What's the most important thing to look for? Obviously, they should be the kind of person you can be in business with. In other words, trustworthy and on the up and up. If they ask you for money, run! Must they be well-connected in the business? It certainly helps, but you could be better off with someone who is hungry and scrappy. Does the agency/management company have to be big? No. My first agent was with a medium-sized agency. I had much more success with my second agent, who was a one person operation. I could go on and on. But after some experience, I believe the most important thing for a representative to have is also what a writer happens to need - perseverance.
I recently received an email from my manager that didn't have much good news, but what I really liked about it was that he is as gung ho as ever. And boy, does this make a difference! I once had a book agent who told me she would tirelessly push a novel of mine for as long as it took. Then she threw in the towel after four rejections. Four! I had a script agent who I really loved, except that she was quick to get discouraged. Yes, some scripts are duds. But there are so many wonderful movies that only exist because there was someone (or a team of people) who wouldn't give up no matter what. And that's the kind of representation we all need. Unfortunately, you don't really know what you're getting until you sign. But if that's the kind of agent/manager you end up with, then stick with them while they stick with you. Because, as Derek Jeter used to say, this not a sprint -- it's a marathon.