I honestly try not to flip off drivers anymore. But I also try to do nothing whatsoever once they go by. I think most any action will result in a confrontation with some drivers. It is difficult sometimes, to simply keep my head down and continue on as if nothing happened.. One thing I found that does sometimes work, though, is to wave and say "SORRY" before they actually get by and have a chance to "express" their anger. I start my wave when I feel them coming out and around, and I look over and mouth the word "SORRY" before they have a chance to do whatever it was they wanted to do. Sometimes it throws them for such a loop that they actually wave back. It's sorta funny, in a strange way.