Wefollow.com is an authoritative directory of people and companies using Twitter. Each user is tagged and categorized.

If you use twitter, visit this site. Then do two things:

• Register yourself. You get to pick three tags – pick ones that are most relevant. If you have done SEO research lately, pick three great keywords.
• Choose people who have categorized themselves with the same tags and follow them. This is a way to be part of topics that interest you. Plus, when you follow, the person is likely to follow you in return.