Dan Zarrella, the "social media scientist," has a post especially pertinent for PR Twitter users: Tweeters don't like to find you talking about yourself. Modesty pays on Twitter, as elsewhere. Don't talk about yourself or your organization (unless actions of your organization are directly pertinent).
Providing links, giving other-centered advice, being a good listener, sounding unassuming are stances that gain attention and possibly increase influence from people on Twitter.
As Zarrella notes of his Twitter research: "The 'Social Behavior' category includes inclusive language like 'we' and "you', as well as language that describes relationships and communication. As it turns out, accounts with more followers tended to be using more social language."