How to use Application Helpers in Ruby on Rails

I'm attempting to post a random testimonial on each page of a site.

I started off by using an application helper like this:

module ApplicationHelper

  def random_testimonial
    @random_testimonial = Testimonial.find(:first, :offset => rand(Testimonial.count))
  end

end

In my view I can reference the method but it's being called on each reference, which makes sense.

I'd like this to be called once on each page view exposing a Testimonial object I can use within the view.

What should I be looking for to do this?