Just last week I was slowly making my way up one of my nemesis hills when a car came by and the passenger yelled, "You're awesome, keep pushing!" At first it scared me so much because I was in a zone and was not expecting to be yelled at, but then I realized that they meant well.

I agree with the others who mentioned the Share the Road stickers and licence plates. Also, I think its nice to chat and give some encouragement when both the car and the cyclist are stopped at a traffic light.