Not a fix, but a survival tip - If your steed starts to shimmy at speed, clamp your knees onto the top tube for all you're worth. That should change the harmonics enought to stop or at least contain the shimmy. I had a speed wobble start at 35-40 mph once, and I thought I was going to die - until I put a death grip on the top tube with my knees. Problem solved, and my heart rate eventually came back down.

The knee-clamp trick is is no substitute for trying the various solutions offered above, but it's a damn good thing to know about, because in my experience, speed wobbles at speed (a) are no fun at all and (b) tend not to announce themselves ahead of time.