1. Post regular Twitter updates everyday. That doesn't mean spam or post them constantly all day long, but there should be enough regularity to let your followers know you're there and don't forget about you. That said, don't talk about anything. Have something valuable to say. If you produce content, share your links. Some may not think it's a good idea just to pump out your own links on Twitter, because it's "about the conversation", but the way I look at is this: if someone is following me, they're probably interested in what I have to say. If I'm writing articles, those are in essence, what I have to say.
2. You should engage in conversation on Twitter. Start conversations that don't necessarily pertain to your own links. Listen to what others are saying, and join conversations with them, regardless of if they are directly related to your brand or not.
3. Monitor your brand. Respond to @mentions (good or bad). Respond to direct messages.
4. Post pictures and videos using services like TwitPic, TwitVid, Yfrog, etc. These things can help humanize your brand and increase engagement with others. They can be conversation starters. People like visuals.
5. Create relevant lists. Create lists of other Twitterers that can provide value to others. You don't necessarily have to, but it might be a good idea to create a variety of lists for subjects related to your niche. If you have a car blog, for example, you might have a list of car brands, a list of other car bloggers, a list for mechanics, a list of auto part vendors, etc.
