Inspired by David Allen Green's How to lose followers on Twitter, I thought I'd share some tips on what to avoid doing if you want to be big on Twitter.
I don't think there's any simple formula for gaining followers. You just need to write "good" Tweets, where "good" is impossible to define and depends on what kind of thing you write about. However, there are some simple things that are pretty sure to lose you followers.
Send a Tweet. Every time you Tweet, expect to lose followers. People will unfollow you when they see your Tweets and decide they don't want to hear more, and the more you post, the more likely they are to feel it's too much. Dense "bursts" of Tweets, in particular, may make people decide that you're taking up too much space in their time line. Tweeting is the only way to gain followers, but unless a Tweet actively gains followers, it will lose them.
Treat Twitter as a social network. It's not. Or rather, it can be, and that's fine if you want to use it that way. But if you care about your follower count, then you're probably not thinking of it as one. Twitter is a microblogging platform, so treat your Tweets like blog posts aimed at your audience, rather than personal "chat" messages. Chat Tweets rarely gain followers... and that means they lose them (see above). It's not an absolute rule, and if you never chat, it will cut you off from important discussions. But chat should not be your default mode.
Spam. There are variousservices that make cookie-cutter Tweets on your behalf. Usually they're a round up of your recent Tweets, or a follower count. In general, avoid these. Your followers want to hear from you, not from a robot, and they want to hear your thoughts, not your Twitter stats. They can be useful, but think very careful before using these.
Be Bilingual. If you speak more than one language, that's awesome. It means you can communicate with more people. But don't try and mix languages on the same account. If half your Tweets are English and half are French, then half your Tweets are essentially just meaningless clutter, for any given follower. Make two accounts, one per language, and you'll get more followers overall.