I've heard people buying Twitter accounts with large followings and then migrating their username over. That's one way - although I don't get that at all.

Other people have done it with Follower farms .. were they join a follower farm site, and essentially everyone just agrees to follow each other.

Both of these are in violation of the terms of service and may get your account suspended/banned.

The best way to grow your audience is to share things worth sharing. Either quotes, pictures, interesting / funny insights, etc.

I'd rather have 1000 true fans than 200,000 bot followers.