What's the right syntax to use Rails `content_for` helper in a HAML context

That depends on what you want to do.

To render the header right away, do:

= content_for :header do
  %h1 Title

To store the content and use it later, do:

- content_for :header do
  %h1 Title

And to use it somewhere in your view(s):

= content_for :header

In Rails < 3.2 you needed to use = yield :header. That is still supported in Rails 3.2 but it doesn't work in helper modules while content_for does (thanks @drewish).