I think the answer partially depends on what data you are trying to display. If the data is static, or related to something predictable, like the date, I would create a helper function and create the data in there. There might be cases where the data you want needs to come from the database though, and for that I would create a before filter:
before_filter :do_stuff
def do_stuff #stuff! end
You should read up on before filters, there are more options that make them conditional and such that can come in handy.
Read more here http://api.rubyonrails.org/classes/ActionController/Filters/ClassMethods.html