I have become an aggressive counter-flasher. This has yielded in some cases new knowledge - that the low beams of a lot of cars these days look like high beams (indicated when they flash back, and it's the brightness of a thousand suns).

For those behind me, I've discovered that my side mirror has an angle where it reliably bounces the beams back. I've gotten more than a couple of drivers to turn their beams down with this method (but they have to be tailgating for it to work, which usually means we're already in an adversarial situation).