This link is a good read for the beginner. It gives many ideas for searches further into preventives. This link only delves into the world of prescription medication and unfortunately ignores alternative and complementary medicines as well as more complex medications for those refractory to the simple meds listed. Still, it is a good introduction to prevention which remains highly undiscovered by many migraineurs.

A good laundry list of medications and some general information. The site would be much improved if each description of a medication included a link to prescribing information and of course the caveat that you should work with your doc to find the meds right for you.