A forward head posture is common with headache pain. A great exercise to help reduce tension and muscle guarding is nodding. This post includes a video on 'how to' treat headache pain through daily exercise.
Sometimes headaches can be relieved by neck traction. Here is a video showing how to do this on someone you care about. Remember to go slow and get feedback.
Here are two muscles that are common with headaches, the temporalis and the superior oblique. This video shows how to massage these muscles to reduce tension and headache pain.