I've rarely been harassed, but one thing I do is to have a prepared response that's safe and neutral, that I can execute without having to think about it much. I simply slow down. Very few drivers will slow down to match your speed. If they're ahead of you, then they can yell into the wind all they want. If they slow down too, then they're perceived as the ones who are obstructing traffic -- guess who gets honked at.
The other is to keep a bit **** eating grin on my face.