I feel that cervicogenic headaches, can be caused by anything from faulty biomechanics of the cervicals to poor crainal circulation to increased strain on the optic nerve to poor mobility through the sacum and lumbopelvic complex. We need to look at the whole body, expecially when headaches are involved.
I think I will begin by talking about biomechanics of the upper cervical spine. Typically one will find a complete disruption of mechanics throughout the c-spine but especailly C0-C3. Always there is some type of compression through the OA joints either one sided or bilateral. C1 will typically shear forward and C2 will appear relatively posterior and C3 will typically be relatively anterior to C2. The rest of the c-spine will do its best to compensate and right the head over the body in order to maintain balance and an appropriate field of vision.
Please do not forget about the importance of the cranial base and its relation to vital blood supply and nerve tissue. You had mentioned working the suboccipital muscles which will do wonders not only in aleviating protective muscle spasm in that region but also in decompressing the craninal base to allow improved blood flow especially through the circle of willis and decreased tension through the cerebellum and brain stem (all cranial nerves leave the brainstem!).
Also, please do not foget the improtance of the dural system and its attachment to the cranium and spine which can (and often does) contribute to increased spinal tension, faulty biomechanics and protective muscle spams.
The sacrum is of utmost importance in the treatment of headaches, as the dural tube inserts on S2. Any dysfucntion of the the sacrum will create a drag on the dural system contributing to a myriad of symptoms including compression of the cranial base and headaches.
Of course an appropriate program consisting of stretching and strengthening and neuromuscluar re-educaiton is critical to integrating the work done above. Address postural faults and muscular weakness. But please remember that each patient is different and unique and needs to be treated that way. Just because two people have identical symptoms does not mean that two identical treatment approaches are going to prove the same effectiveness. The therapy we pracice should not a cookbook that states do A, B, and C for X problem. Each patient needs to be evaluated objectively without bias in order to diagnosis the root cause and provide the most effective needs for that situation. (sorry about the rant)