Some people sign "100 year."
What I use and most of my colleagues is a "C" handshape that is moved in a quick small circle (clockwise to you, counterclockwise to your observers).
Note: This sign is usually only recognized in context and it is helpful to use a bit of lip movement. I showed it to several of my deaf friends in isolation and they had no clue what it meant. As soon as I put it into a sentence most of them recognized it.
Animated version: (Autoload: About 1MB).
A variation of the sign century is to instead of circling it you can shake it side to side a couple inches.
American Sign Language University ™ ASL resources by Lifeprint.com © Dr. William Vicars