To start, I found some great websites about migraines. The U.S. Department on Health and Human Services has a great article that answers common questions about migraines. The May Clinic has and article that discusses symptoms, disease process, triggers, complications and treatment. Livestrong also has many resources related to migraines and headaches. See the resource list below.
Migraines and headaches are triggered by many sources. The practice of yoga can ease the sources, preventing or lessening headaches (in strength and frequency) and ease symptoms once a headache starts.
Benefits of Yoga:
Breathing (Pranayama): Yoga practice includes time for breathing which may relax our body, releasing tension and bringing us to a quieter place.
Mudras: Yoga includes mudras, which are hand postures, that relax our shoulder/neck area releasing tension and easing pain. The "gesture of the great head" is a mudra that along with breathing may ease headaches.
Self Awareness: As we participate in yoga, we become more aware of our bodies - how they move, react and feel. With this increased awareness we are better able to sense the signs of an impending headache. We can then participate in preventative actions. We will also be able to tell our medical practitioners more about our headaches - how they begin, and what triggers them.
Relaxation: Relaxation is an important part of preventing and relieving headaches. It eases tension, allows us to power down and makes us more resilient to life's challenges. Svasana, resting pose, is a great example of relaxation in yoga. During svasana we are turning ourselves away from our daily routines, in a quiet and darkened room giving us a deep sense of rest.
Posture: In yoga we learn posture and body alignment. Improved alignment and relaxation of shoulders, neck and head will ease tension and put less pressure on those muscles that otherwise might cause headaches.
There are a few things to be aware of in yoga practice that might instigate headaches or make them worse. When in a yoga posture, avoid hyper extending the neck. Instead extend out through the crown of the head. "Cobra" is a good example of a pose where there is a tendency to hyperextend the neck. Some references said to avoid downward facing poses (downward facing dog) during headaches, others said it was okay. So it's probably good to listen to your body. Yoga as Medicine suggests being careful when using pranayama (breathing) with "long retentions of the breath and with fixed ratios of time spent inhaling and exhaling." They also stated using vigorous breath practices like Bhastrika breath could be problematic.
Poses: Restorative and more meditative postures are most helpful with headache prevention. For example, "feet up the wall" with a bolster under the hips. It may be hard to jump right into a practice of restorative poses, after a busy day, so it is suggested to do more active asana first, then move into a relaxing, quiet practice. When a headache is present, doing a quiet, meditative practice may be best.
In the end it is important to listen to your body, work with your medical practitioners, do a little research, find a yoga teacher you feel comfortable with and work out what is best for you. Below is a list of references that you might find helpful.
"On a Roll" www.yogajournal.com - how to 'melt' tightness and headache relief
"Migraine - Frequently Asked Questions" www.womanshealth.gov - Dept. of Health and Human Services
Yoga as Medicine by Timothy McCall MD www.yogajournal.com
Mudras for Healing and Transformation by Joseph and Lillian LePage