Put in a LOT of time and effort. Listen as much as you speak. I don't mean this as an obnoxious answer but that's the only real, sure-fire way to gain a following on Twitter. Provide value in a conversational manner and you will start to see your users naturally climb.
Utilize search.twitter.com to camp on terms and subjects relevant to your industry and engage with those users. Favorite their tweets, reply to them (don't hard sell anything on first interaction) and if they seem influential in your niche, follow them.
Use followerwonk to find highly engaged members and mimic their activities, or find ways to engage with them and their followers.
Set up private lists to follow your peers and competition to see what content is providing high value and engagement.
All in all just keep at it and stay active, if people @mention you or use your brand on Twitter, do NOT ignore them, engage, engage engage, good luck!