It is now believed that the dysfunction in the autonomic nervous system is the primary root cause of migraines and dilation of blood vessels is a result of this dysfunction. So anything that soothes the autonomic nervous system will benefit migraines. However, as a migraineur myself I have found that deep breathing exercises and mindfulness meditation are the two most beneficial and if you learn how to do it accurately, you can actually turn off the migraine completely.
On the other hand, prevention of a migraine is always the best practice, and this is accomplished most effectively by becoming aware of your primary triggers and avoiding them. Some of the most common triggers include food and environmental sensitivities, monosodium glutamate, changes in weather, dehydration, exercising too strenuously, cigarette smoke, hormone imbalances, caffeine, skipping a meal (which causes low blood sugar), artificial sweeteners like aspartame and pickled or fermented foods.