1. Have it be comprehensible.
Shrtning vrythng is cnfsng. 140 characters is vital for Twitter, but people need to easily understand your message.
2. Encourage participation.
Whether that's tweeting at someone or calling your audience to action, it's important to get your followers motivated.
3. With everything you write, make sure it's worth everyone's time.
Sure it's only 140 characters, but there are just some things that people do not need or want to know.
4. Respond to tweets often.
Whether it's just a simple "thanks for the follow" or answering others' questions, it shows that you/your organization values relationship forms of communication rather than the out-dated 1-way communication model.
5. Tweet about different things.
Keep your Twitter account interesting. Just because you focus career-wise in one field doesn't mean you can't tweet about other things that interest you. And chances are, you can relate most things back to what you're "known" for.