This is the subject of many a spammy autofollow sleazebag, I know. But there is no point in Tweeting if nobody’s reading. Relationships are what it’s all about, and that starts with following and being followed.
Quantity is important, but so is quality. The two are not mutually exclusive. You can have both.

You can have quality interactions with only so many people, and after that what you have is audience to broadcast to. If that many people want to hear what you say, good for them (and good for you, too).
Use Mr. Tweet to find other people on Twitter in your marketing niche and follow them. Many will follow you back, and they’re relevant to you and your existing followers.

Engage in conversations with top tweeters. Top tweeters are people who tweet a lot and who also have a lot of followers. The more followers a person has, the more likely those followers are to notice you, and the more likely they are to follow you.
Ask your followers to recommend you to their followers. This one takes a little guts, but if you know the other person well enough, there’s nothing wrong with it. Most people are happy to do this for you, they just didn’t think of it. You can do this via reciprocal arrangement through DMs.
Ask your blog readers to follow you on Twitter. And along the same lines, put a widget on your blog that has your latest tweets and/or the count of your Twitter followers.
Put a link to your Twitter profile page in the biography you include when you submit guest posts to other blogs (which means you should be guest-posting on other blogs).
Leave comments on blogs that let you include your twitter name, such as TwiTip by ProBlogger Darren Rowse.
Sign up for TweetBeep, a service similar to Google Alerts, but for tweets. This will help you go where the conversations are happening that you’re interested in. Engaging with others will get you more followers.

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