I use the method on http://www.twitterdoctors.net/ and more than 70% of my followers are now doctors, which is useful for the occasions I do tweet about new features and stuff related to my healthcare start-up.

For those interested, I am in the process of re-writing the site (dev version at http://www.twitterdoctors.com/) – to automate the process, try the wizard out, it uses twitter's new event listeners.