A lot of people think that it's vital to get as many Twitter followers as possible. But as with so many other things, it's not the quantity but the quality that counts.
That's why you should focusing on finding tweeps in your niche. Also, seek out those who aren't just selling stuff, but who are actually using Twitter to express ideas and opinions and link to their blogs and others. Also, see how often they mention other tweeps and retweet.
If they do both these things quite often, there's a chance that they'll also mention your tweets or retweet them, particularly if they're good. That will help expand your following further. (Needless to say, it's wise to return the favor from time to time.)