Johannes has a pain in the neck.
As someone who has had a headache (the same one, to varying intensity) for approximately 12 years now, I am totally with Johannes on the importance of posture, neck, back & abs strength to fighting back.
My headache began one week after I started a job which had me at a desk the whole day. I will not bore you with the countless fixes I tried over the years (suffice to say Neti pot). It's only the fact that I now work from home, with the flexibility for work location, integrated stretching/exercise, G&Ts etc that has given me control.
I do all the same neck stretches that Johannes describes. Ultimately though, for myself, the only thing that can effectively kill a headache in its tracks are pressure point massagers that I found in the Tokyu Hands department store in Tokyo (the Japanese take stress reduction very seriously). If you have neck pain and find yourself anywhere west of San Francisco, make a trip to Tokyu Hands.