In the beginning of your Twitter campaign you can click "find people" and via Gmail, Yahoo, or AOL you can find friends who already use Twitter.
Find and follow the leaders in your niche by scouring the many Twitter user directories. Be prepared to spend time observing these leaders and the various social orders within the user base.
These observations will help you to find the leaders in your niche.. Pay particular attention to these trendsetters/influencers. Befriend as many of your competitors as possible, but do not forego the small guy as they tend to be quicker to repost your messages. These relationships should be nurtured over time and a few may well lead to some kind of joint ventures in the future.
Another simpler way of getting followers fast is simply promoting your Twitter on your blog, Facebook fan page or email lists of people who are already following you. Tell them that they can get the latest updates and news (as well as special offers) by following your Twitter.
In short, join the conversation and get involved in your community! That is the fastest way to build a huge following.