I'm partial to getting DVDs and books from Amazon so I can leverage reviews to have a better chance of getting something worth my time and money. That's how I found these, which I can recommend after trying them:

* Posture, Get It Straight! Two Disc Set by Janice Novak

* Fixing You: Neck Pain & Headaches by Rick Olderman

* Pain Free at Your PC by Pete Egoscue

Some others that are well reviewed, but that I haven't cracked yet:

* The New Rules of Posture by Mary Bond

* Why Does Working @ My Computer Hurt So Much?

Also, if you have droopy shoulders and/or forward head, I recommend the "Brugger Relief Position" several times a day, which you can look up on the web and youtube.

HTH