Twitter is a social network. If you want to earn more followers, you .. have to network. Follow the people you want to follow you. Try to communicate with them using "@." ReTweet (RT) their message. Post engaging and thoughtful content for your existing followers and they will respond and/or retweet your thoughts to their followers. And believe it or not, an interesting or attractive profile pic helps.
In addition to what InsightsDigital reference, Mashable has a pretty good twitter guidebook.