http://stackoverflow.com/questions/9965111/rspec-shared-context-and-include-context-for-all-specs

beer_spec.rb

require 'spec_helper'

describe "Brewery" do
    
  it "makes good stuff" do
    beer.should be :good
  end
  
  it "makes not too much bottles" do
    @bottles.should == 10
  end
  
  context "when tasting beer" do
    before(:all) do
      @bottles -= 1
    end
    
    it "still produces good stuff" do
      beer.should be :good
    end
    
    it "spends some beer on degusting" do
      @bottles.should == 9
    end   
  end
end

global_helpers.rb

module MyApp
  module GlobalHelpers
    def self.included(base)
      base.let(:beer) { :good }
      base.before(:all) { @bottles = 10 }
    end
  end  
end

spec_helper.rb

require 'global_helpers'

RSpec.configure do |config|
  config.include MyApp::GlobalHelpers
end