Barry Deen has a very helpful article on building a Twitter following on his blog at:
http://www.webstrategyblog.ca/build-your-twitter-following-fast-spam-free

Here's the post:

Build your Twitter Following Fast & Spam Free!
by Barry Deen on March 25, 2009

I’ve been asked several times how I’ve built my Twitter following so quickly - so behold my secret to building it quickly, and spam free!

Note that many people may disagree with my tactics, and if you’ve read my previous post about “Grey-Hat SEO” - I would consider my method “Grey-Hat Twitter Marketing”. The thing I know for sure is that these tactics work and have driven great traffic to my various websites and has helped me build out my social networks.

After one month on Twitter, here is my Twitter Counter Chart:

I went from 0 followers to 1,349 in a period of approx. one month.

Here is a break down of the stages of growth:

Artificial Growth:

I use a tool called Twitter Perch (you may remember or find more information on my vlog) - it can be found here: http://www.twitterperch.com.

I’ve entered in keywords relating to my sites (Web Strategy, Web Design, Social Networking etc.) and the tool automatically follows people who have those key words in their tweets. It allows you to follow 20 people per day per keyword. For roughly every 4 people you follow, one will follow you.

Now Twitter has an “anti-automation” script that tries to balance out how many people you are following. If you are following 2000 people but only 500 are following you, Twitter will not allow you to follow anymore people until your following catches up.

You’ll notice in the middle stage, my “organic growth” was kind of flat (growth coming only from my websites) - so I needed a way to unfollow the people who haven’t followed me back.

This is where Twitter Karma comes in (http://dossy.org/twitter/karma/)

This tool allows you to bulk unfollow people who haven’t followed you back. This way, you can reduce your following count - thus allowing you to follow new people who may in turn follow you back.

Everytime you’ve reached your maximum (I’ve noticed twitter has a plateau around 2,000 follows) - you’ll login to Twitter Karma and “clear out” the people who you are “only following”. Repeat this process and you’ll see a huge increase in your following count!

Note: that I do not endorse using this method if you plan on simply spamming out your links. Most people who you will follow will have low following/follower counts so they will pay close attention to your updates. If it’s just spam, you are not likely to be added. If you post legit questions, comments and funny updates - and have a nicely designed twitter profile; you’ll increase your changes of getting more followers.