First though, I want to remind everyone that QUALITY matters more than QUANTITY. Unless you're racing to a million followers as a publicity stunt, don't loose focus of your real GOAL: getting more sales! Always strive for quality AND quantity.
Ask Your Followers to Re-Tweet Your Tweets
Teach your followers what re-tweeting is (basically, it's the Twitter equivalent to email forwarding) and ask them to re-tweet your tweets. This way, your tweets will spread to their followers, who then may follow you as well.
Add Your Twitter Link to Your Site, Blog and Email Signature
The more people who see your link, the more followers you'll get. 
Use HashTags
A HashTag is a keyword preceded by the pound sign. For example, people talking about gardening might include "#gardening" in their tweets so that others talking about gardening can find them. Here's a website, where you can find the most popular HashTags. Using HashTags is a great way to ensure your tweets get found, leading to more people following you.
Be Part of the Conversation
Following people isn't enough. You have to actively participate in the Twitter community. Help your followers by sharing tips, cool things you find, etc.
Follow the Authorities in Your Field
Put together a list of the people in your industry you admire. Then follow them on Twitter, re-tweet their tweets, and follow people who follow them. If you're sharing great content on similar topics, some of them will follow you back.
Go Multimedia
People love photos, videos and music; include them in your tweets.
Follow People Talking About the Things You Talk About
Use the Twitter search to find people talking about your field of expertise. Follow them. Most of them will follow you back.
What do you do to get more and better followers?