I volunteer with Citizens' Climate Lobby, which I think is the most effective action an individual can take, probably. The best individual action is to put pressure on politicians for collective action. We have to fix the rules of the game -- right now, it's way too cheap to pollute.

One important thing we do is to call Congress regularly and express our support for a carbon fee & dividend policy, for example: https://citizensclimatelobby.org/senate/

And of course, there's lots more to volunteering with CCL if you want to get more involved.