Engage with your followers. Make them feel welcome. Reply to their tweets, retweet or favorite.
Live Tweet events and use the offical hashtag.
Share funny things that happen in your daily life.
Host a Twitter giveaway. Rule: They must be following you to enter.
Get in on discussion.
Use relevant hashtags.
Make connections with important and relevant people.
Buy your followers.(Guess what! They aren't real people... Meaning they don't give you retweets or favorites. Meaning it's obvious that they're fake. And you just look silly.)
Be an annoying fangirl. It is fine and great to be a fan of something, but there IS a difference between being a fan and being a crazy, obsessive, creepy fangirl.
Tweet too much. You don't want people to get annoyed and mute you, or worse.... unfollow you.
Post spoilers for movies and tv shows. Yes, we all like to talk about current movies and tv shows that we like, but there IS a difference between talking about something and ruining it for other people.