I second just giving them plenty of room as you pass (if it's safe to). Maybe a thumbs-up would be nice. If you want a bumper sticker, they may be purchased here (or get one free with a Bicycle Coalition of Maine membership).

If I may be allowed to go off on a related tangent, I had another cyclist yell some encouragement at me the other week. We were approaching each other on a primary road; I took a left turn to a side street before he reached the intersection (I didn't cut him off, there was plenty of room), after moving to the middle of the road and signalling for at least 50 feet. As I was riding down the side street, I heard him yell "Nice use of turn signals, John!" as he passed on the primary road. It wasn't in a sarcastic tone of voice, as it may come off here in writing; he sounded sincere. Secondly, he obviously knew me, but I have no idea who it was!