1) Follow local.
Follow the right people. Newbies on Twitter almost inevitably mass follow celebrities and public figures. Forget it. Follow local people and engage within your own community. Twitter is about making your real-life community smaller. Follow people you have opportunities and would want to meet with in person.
2) Start and join conversations.
@ reply to other people's tweets, adding to their comments. The best interactions over Twitter come from random conversations about topical, obscure, or inane things. I often find my most commented or replied to tweets are little things I'm randomly thinking about or witty anecdotes. These little gems can relay true insight and add to a cult of personality. Ask questions and reply to others.
3) Actually read tweets.
Make sure to read other people's tweets. If you want to gain a following and actually engage with others, take the time to read others' updates. How can you expect anyone to read what you have to say if you don't read what others do? Twitter and social media in general is about engagement.
4) Retweet and be retweetable.
Good tweets are inevitably retweeted. Retweet things you enjoy. Spit out some valuable information in unique, fun ways, spreading your thoughts and ideas. If you want retweets, don't use up all of the 140 character limit. Depending on the length of your username, leave 15-20 free characters for retweets. Keep the "retweet to win" contest entries to a minimum.
5) Be useful.
Tweet or retweet interesting, useful, insightful, or amusing links, articles, photos, videos, etc. Don't just sell yourself, your brand, product, or company. It's all about adding value to yourself and your online presence.