I’m not a joiner, but I belong to three non-partisan organizations concerned with human rights and have combating censorship as part of their expressed goals. Their mission statements can be found on their websites.
Amnesty is one among several organization devoted to protecting human rights, and among those rights is freedom of expression.
CCLA (Canadian Civil
Liberties Association): http://ccla.org
This is the Canadian counterpart to the American Civil Liberties Union. They are quite willing to support freedom of expression, even when doing so is unpopular or even when it is being exercised by organizations that are clearly lunatic or are expressly devoted to suppressing human rights. An example is defending the right of the Neo-Nazis to parade. They obviously have taken to heart the dictum “I may hate what you have to say, but I will still defend your right to say it.” They have guts—and principles.
PEN Canada: http://pencanada.ca/about/
This is the Canadian branch of PEN, committed to protecting the rights of writers to publicly express themselves without persecution by governments or anyone else. Such persecution, legal or illegal, ranging from financial intimidation to imprisonment, is only too common.
Another way to combat censorship is not to self-censor when talking about it with others. However, it is a surprisingly (and dismayingly) way to make enemies—even among the well intentioned.