To fend off an approaching tension headache, or for relief once it starts try this variation of Advasana:
Lie face down with your forehead on the floor. Clasp your hands and cradle the base of your skull, with your elbows resting on the floor. This may be enough to begin the easing process. Be aware of your breath, and as you inhale push the back of your skull into your hands keeping your elbows on the floor but lifting your forehead a little way from it. Feel the gentle easing at the top of your spine. Rest your forehead back on the floor as you exhale and repeat until the tension eases.
What favourite (free) ways do you have of preventing or relieving headache?