Firstly, don't pay for them or do a mass 'follow the unfollow'.
Try great content, tweeting pictures, helping people out, joining conversations etc - the growth will be slower but a better quality of follower.
If your content is good, it will get re-tweeted.
Try mentioning others in tweets. If you are saying something good, chances are they will RT and your tweet will then be seen by their followers, some of who may then follow you.
Obvious one is to add your profile name on email footers etc and to link to your profile on any other social media profiles.
Take a look at top tweeters to see what sort of thing they are saying - you may be able to learn from it.
Good luck