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).
Haha I've also angled my side mirror out of my eyes, which incidentally is back towards the car behind me. I of course angle it back if I need to change lanes, but it's such an annoying thing I have to do just to see the road ahead of me.
At this point I put full blame on car manufacturers and lack of government regulation and enforcement. Lights will keep getting brighter because lights are getting brighter. It's a death spiral.
My rear view mirror does this, I wish my side mirrors did too. Although recently I've noticed some cars headlights can even pierce my rear view mirror's polarized dimming. It never used to be a problem in the past. I've seen the difference when drivers turn their high beams on and off. It always did a great job against driver's brights including large trucks. But occasionally there's now a vehicle with the light of a thousand suns that is too bright for the auto-dimming.
… or a steerable corner-cube array or retroreflector prism. Steerable in that it needs to slightly redirect its reflection to above the light source—to the windshield area of the offending vehicle—rather than exactly back to the light source.
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).