First of all, I must admit that this was not written by me. This is an entry I read from one of my LinkedIn associates, Geoff Evans. But he does such a nice job of explaining it that, rather than trying to put it into my own words, I wanted to present Geoff's own words and give him full credit for them. The following is from an entry of his I saw on Linked In:

It seems that the most often recommended strategy for growing your Twitter followers is to simply follow others.  Many people have setup “auto-follow” or feel the courteous Twitter thing to do is to follow you back.  How often do you see a Twitter guru who is following as many (or more people) then are actually following him/her?  This isn’t someone who has necessarily brought value and therefore has established an enormous quantity of potential clients.  This is someone who has probably played the numbers game.
The strategy of striving for quantity is purely a marketing initiative.  However, marketing becomes irrelevant if you ultimately have no basis on which to ever convert the potential clients into actual clients.
In reality, long-term Twitter success isn't about having a thousand followers.  Real business success will only come from the individual relationships you build by participating in the conversations and sharing your ideas.
If you choose to pursue a strategy of gathering a quantity of followers then you'll likely end up being followed by people who are seeking the same thing. Unfortunately, your message will quickly get lost amidst the thousands of voices all busy broadcasting their own marketing messages.
I believe the greatest Twitter success will come from a process of growing your followers organically.  Connect your Twitter identity to your email signature, your website, your blog, newsletters, etc.  Yes, this approach will take time to grow.  However, people who use Twitter WILL follow you if they feel that your message is interesting or helpful.  If your only goal for earning followers is to create a platform for selling your products and services then you will lose these followers as quickly as you gained them.  It only takes the simple click of a button to be “unfollowed” or “blocked”.  And notice that I chose the words “earning followers”.  To be followed with any meaningful result on Twitter means you have to earn the right to share your thoughts and ideas with others.
Also, consider leveraging Twitter, not just as a vehicle to broadcast your own message, but rather as an effective tool for listening.  Listen to the types of conversations that are taking place in Twitterville.  I often use Twitter to help me identify future blog topics. If a certain topic is trending (and relevant to my business) then people are more likely to be intrigued by an insightful blog that relates to this topic.
In summary, I would highly recommend that you make the choice to follow people because you're interested in what they have to say not because they'll auto-follow you back.  Otherwise you'll quickly grow tired of Twitter and you’ll miss out on the real opportunities.