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?