My chiro suggested we desk jockeys (his words, not mine - he's such a kidder...) hang your head backwards off of a bed for five to ten minutes daily to decompress the spine. I picked up a posture pump (ebay it for like $60) which is really awesome at pushing into the back of your neck and man, it's great for those headaches. Basically you're dealing with referred pain from tight/short muscles via poor posture or simple fatigue from holdin' yer coconut up.
The poor posture of forward/hunched shoulders stretches the trap/rhomboid complex and makes it weak and tired. It basically goes into spasm and until you remove those ischemic areas (aka seized up) you're in for some pain.
Good luck!
Grim
EDIT: NY Muscle makes an excellent point about ART. My massage therapist does that and although painful, it's beyond helpful in remaining balanced and with full range of motion. Yep, the pros all do it.