It is helpful to reflect on the impermanence of the headache, because often when we have any sickness or pain, we start to project how long it is going to be that we are going to have this pain. It is very helpful to reflect on the impermanence of the experience so that we don't start projecting, getting concerned and worried. Then we can deal with the situation in the best way by observing the headache, trying to observe the unpleasant physical sensation. Certainly, with a headache, it can be very helpful, depending on the severity of it, to lie down so that you are not doing anything particularly strenuous at that time, observing the sensations, and trying to feel Compassion for yourself. Then, it is also helpful to do a bit of D/D or universalizing of the particular experience so that we don't suffer with it so much, understanding its universality, and allowing ourselves to accept what is there.
The D/D is a method we can use after we feel Compassion for ourselves and relax the resistance in the mind towards the sensations. This is how feeling Compassion for ourselves helps. First, we just observe the sensations moment to moment and try to bring up Compassion for ourselves. Then, we think of other people in the world who may be having a similar experience, trying to feel Compassion for them, and then go into other situations where the headaches are more extreme, perhaps to people who have migraines which last for three days.
Certainly, the worries projected into the future - "how am I possibly going to be able to deal with and do everything I need to do if I have a headache?" - these are thoughts of self-pity. We want to try to stop ourselves from falling into that so that we can deal with just the moment-to-moment experience, so that we are not dealing with all the worries and projections on top of the experience. We observe it, try to feel Compassion for ourselves and think of others in the world who have similar experiences or even worse experiences.
I have friend who has a brain tumor and lives with it, defying all odds. By thinking of these types of experiences, we don't feel sorry for ourselves. Then we get out of what I call the "small mind," being concerned only about myself and thinking that the experience I have is terrible, and instead get into the bigger mind of Compassion that helps us to deal with difficulties without suffering as much with the difficulties. Sometimes when we are into self-pity, the smallest difficulty or Dukkha seems big, but when we get into Compassion, the biggest difficulty gets smaller because we understand its universality and that it is not just me; it is also others, and this is the human condition. If we have a tendency towards headaches, then it might be helpful at other times, when we are not having a headache, to reflect on the physical and mental Dukkha that human beings are subject to in the course of their lives, so that we accept that Dukkha does arise with more Equanimity.
Sometimes, when I have a long headache, I also use other types of techniques to deal with it. Sometimes, reflecting on confidence-inspiring objects can actually help us to concentrate the mind so that we are not so aware of the body and we are in a more joyful type of mental state. I have had the experience where, even though the body had pain, I would concentrate the mind on a confidence-inspiring object, such as reflecting on the Buddha, the Dhamma or the Sangha; good Kamma; our good past actions; Generosity; and Sympathetic Joy, which help to bring up a joyful mind, even though we may be ill with some type of physical Dukkha arising in the body.
Learning how to rest the mind from the pain and learning how to accept the pain can be very helpful. Certainly, working with unpleasant physical sensations in our practice is very helpful for dealing with any type of physical Dukkha that arises during our life. It helps us accept it more easily because, if we have the tendency to only want pleasurable experiences to arise, then when Dukkha arises there is more of a chance that we will reject it and push it away. We have to see both sides of life; not only is there pleasure, but there is also pain; not only is there gain, but also loss; not only praise, but also blame; not only fame, but also obscurity. We are going to be dealing with these dualities throughout our life and we have to learn to live with the pleasure and pain that come from having this body and the need to support it.
On the practical level - that is, dealing with pain when we have it - you can also try, if you are having pain frequently, to look into the possible cause of it. So we don't just deal with pain, but look into the possible causes in our life that may be affecting our physical well-being. If we have some physical misalignment in the body, it may be helpful to deal with that so that the headaches don't come so often. Certainly, I have benefited from chiropractic treatments so that I don't get many headaches any more, certainly not as frequently as I used to have 7 or 8 years ago. Sometimes we also have to look into our diet and our posture. This goes back to the Seven Suitable Conditions. Suitable food and suitable posture are part of the 7 suitable conditions. If we haven't got suitable food or a suitable posture then we may have physical Dukkha arising.