Both magnesium malate and kudzu are effective in preventing migraines if taken on a regular basis. They prevent the initial vasoconstriction that precedes the full blown migraine. Do not take calcium with the magnesium since calcium antagonizes the magnesium and constricts the blood vessels.
You should also keep a migraine log to look for triggers for your migraines. Common triggers include dietary amines, smells, bright lights, hormone disturbances, and allergies.