Gaining followers on Twitter usually requires a reciprocal relationships where you follow back those who follow them.
The key to the art of follow back is to find quality people to connect with in the first place. When I mean quality, I mean Twitter users who are in your target market and interested in a reciprocal relationship.
Follow Back Twitter Tip
When you want people to follow you back observe what their past history is like. The fact is if they are following less than 100 people they will not follow you back. That means that you need to sometimes actually look at potential connections profiles to determine if they are a good fit for you.
Update: The great thing is that Twitter has made this easier than ever with the website they came out with in the fall of 2010. You can now easily look at the basic profile information of different followers without having to go onto their profile page. A small brief summary that makes life easier for finding the right people to follow.
In the end the art of the Twitter follow back is about looking for quality over the quantity. Once you find the quality, the quantity will start flooding in.