Why do I have a twitter?
To stalk agents.
That honestly is my reason, and I'm not the only one. No one follows me on twitter, and I don't tweet that often. But I do read agents tweets.
Why? To get to know their likes and dislikes, to learn more about their lives and keep up with their submission policies. I'm not querying yet, but I will soon, and when I start I want to be able to know a bit about who I'm querying.
Agents like queries that are personalized. If you send out a generic query to twenty or thirty agents at once, they won't take a second look. On the other hand, if you quote something they said in an interview, or say "I'm querying you because [x, y, and z]" they'll know you've put some thought into it. They also love when you reference their recent successes--they're very proud of the books they've chosen to represent, and if you've read them and appreciate them (and if you think yours is similar) they want to know.
So that's my way of keeping up with agents. What do you do? How do you find agents that you think would be a good fit?