I just wave, no smile. I think it lets them know I acknowledged them and that I'm not going to change what I'm doing, because I'm not. I can count the number of times I needed to pull over in my 35 year career as a cyclist on one middle finger. And that was just because the idiot motorist refused to pass, not because they really needed me to pull over. I have been driving that long, and I've never seen a cyclist that I thought should pull over to let people pass.