add_before_filter_mocks_to_existing.rb

describe SomeController do
  before(:each) do
    # exisiting mock setup
  end
  
  # add to mocks, placement after before is key
  it_should_behave_like "a before filter thingie"

  it "blah"
end

authentication_helper.rb

shared_examples_for "an action that requires a user" do
  before(:each) do
    # setup
  end

  it "should redirect to login path if user is not logged in"

  # etc...
end

some_controller_spec.rb

require File.expand_path('/path/to/authenication_helper')

describe SomeController do
  it_should_behave_like "an action that requires a user"

  describe "when listing somes" do
    it "blah"
  end
end