Who do I follow on Twitter?

I have been contemplating my twitter rules of late. Specifically, what makes me follow people?

For people who aren't already following me, it's simple: I keep an eye out for those who are engaging in interesting conversation with tweeps I already respect. I'm not interested in celebrities with big followings, because, for me, Twitter is about conversation. If I want a one-way exchange, I pick up a newspaper.

When someone I don't know follows me, though, what makes me follow back? I reckon it c...