As always, Sheldon Brown has the answer.
From
http://www.sheldonbrown.com/carapace.html:
I never wave, because a prankster put superglue on the palms of my gloves.
Now the gloves are stuck to my handlebars, and it's very dangerous for me to
try to get my hands out of them while I'm in motion.
I do wink, but since I wear reflective sunglasses that make me look like a
bug, people can't even see me winking.
If the oncoming cyclist is riding the same kind of bike I am, I will raise my
left eyebrow 3/4 of a millimeter.
Carapace Completed Umber
Beverly Hills, California