Would you like to live in a country where there's no such law as blasphemy, where state schools cannot offer religious instruction, and where there is no offical state religion? No, this isn't an advert from the French Tourist Board, it is an advert of sorts though.
Relatively few reasonable people (rationalists, humanists, secularists, etc) are members of reasonable organisations. In some cases this may be an active decision, not all reasonable folk feel the need for organisations that promote their worldview. However, I suspect that many people are not members for other reasons. Perhaps they don't know that such organisations exist, or they don't understand what they do or they've just never got around to it.
I've just renewed my membership of three UK-based reasonable organisations because I do think they are valuable. It only took me a few minutes per organisation to subscribe online (links below). I think it's well worth the money. You can also make additional donations to further support the work of these organisations.
The National Secular Society and the British Humanist Association do a great job of providing a reasonable voice in UK politics. Recent examples have included opposition of the religious hatred bill and government funding of faith schools. The more members these organisation have the more weight their activities will have.
The Rationalist Press Association publishes New Humanist Magazine. I think it's really important to have a print magazine that provides a reasonable perspective. A New Humanist subscription is included with membership, or you can subscribe to the magazine without becoming a member.
Of course there are reasonable organisations around the world. But I'm afraid you'll have to go and look for those yourself ;-) There's one international organisation that I will mention though The Brights. As you may of heard, the Brights are trying to get the word bright recognised as a generic term for those with a 'naturalistic world view'. It doesn't cost anything to register as a Bright, but you can make a voluntary donation if you like.
Go on, do it now.