Here is a great story from CIO.com that details how to launch your presence on Twitter. It includes a definitive reading list, examples of how others have done it.
Also, read the reader feedback about launching your presence for smaller organizations and companies. They refer you to TwiTip which provides a bevy of information you can harness for your own agency.
NCE Social Media 
Saw this on the front of CIO the other day, but never got around to forwarding it.