Rails - Jul 2009 - 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?