I agree with those who say "look like a cop". A white BMW, black jacket and a white helmet has certainly worked for me. Sometimes it's a little OTT and the car drivers overreact and actually swerve out of the way when they see me coming (what's the American term - lining it?). I always give a wave in thanks. I like the European "stick your foot out for thanks" method - the first time I did it I wasn't ready for the wind-force on my waving leg. Since we drive on the other side of the road here in the UK, it's usual to wave your left hand in thanks as you wizz past.