Excessive salt and sugar are common causes, along with wine, hard cheeses, and all preservatives. I get migraines and I take gingko every day as prevention, and usually take aleve during an attack, but feverfew is a "natural" remedy that a lot of people say works.
It all depends on how willing you are to stop compartmentalizing things into "natural" and "unnatural". There is only what is, and I'll tell you that migraine is as natural as bald eagle blood and organic as heroin, and I don't want any of those things in my body. I recommend aleve, all-natural and hot off the assembly line.