calling content_for from a controller

i've got a helper method that calls content_for to get all of the content for a sidebar. it's kind of annoying though because i'm having to go into each template file and add <% render_sidebar %>

it'd be great if i could add some before filter in the controller and specify what actions i want the helper method to be called but my controller doesn't like me calling content_for inside of it. what's the best way of doing something like this to avoid the repetition of having to add duplicate code to all of my templates?