In reply to blanchie14c: I think they're genetic in my family - my maternal grandfather, my mother, my sister and I all get them. I suffer the least of the four of us; with my main warning sign being a dull-increasing-to-crippling headache, normally above the right eye. Untreated this progresses to nausea in about two hours; I've never got to the loss-of-vision point. If I can hit it early enough, Ibuprofen (more effective than paracetamol for some reason) will sort it out, otherwise sumatriptan or zolmitriptan and, bizarrely (we think that it's an ingrained placebo effect from years of experience, unless it's just calming), Schubert's Trout Quintet will also help (very useful at work, ibuprofen and the Trout on my iPod will stop me from needing to head home sick)!
Ironically, given the number of people on this thread who have caffeine as a trigger, lack thereof is one of my main triggers (i.e. caffeine withdrawal symptoms for me are headlined by a migraine). The other main ones are too much time unprotected in the sun, lack of sleep and dehydration (net effect = 50% chance of migraine on the first evening of a multiday climbing trip
, no matter what I do, unless I vigilantly hit the tea on return to the hut and have worn a hat/helmet if sunny).