My heart's in it, but I feel like Charley Brown whenever I've tried to wave to cyclists while I was driving.
One day, I saw one coming the other way and I very gently tapped the horn. He jerked and his eyes popped out like he had been shot.
One time, I pulled alongside a group of about 15 cyclists and rolled down my passenger window to speak to them. I called out and said some friendly things, like "Where are y'all headed," etc. They all ignored me like I wasn't there. I figured they thought I was going to give them some "advice" about riding on the sidewalk or something, and maybe that's why they ignored me.
I guess the best thing is to do like some people say, give them lots of room and pass slowly.