The ultimate act of advocacy is: RIDING YOUR BIKE!!!
The more people that ride, the more people will accept cycling as a viable form of transportation. Particularly when they see cyclists obviously using their bikes for transportation with panniers or what not carrying their stuff to work, or home from the grocery.
As a carfree, full-time bike commuter, I enjoy answering questions from co-workers, neighbors, etc. about my bike commuting. Many had never previously given it a thought (that one could use a bike for transportation). While most will never take up bike commuting, just the fact that they now realize that some do helps to promote acceptance on the roadway.
From time to time, I have people from work that I do not know, seek me out to ask questions about bike commuting or cycling in general. Everyone of our 3,000+ employees seems to know about "that guy that rides to work." That's saying something.
Dave