Always use simply

require 'spec_helper'

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If you mix it up like

require 'spec_helper'
require File.dirname(__FILE__) + '/../spec_helper'
require File.dirname(__FILE__) + '/../../spec_helper'
require File.expand_path('spec/spec_helper')

the file will be executed each time, since Ruby (at least 1.8) identifies it simply by the string you passed to "require".