configure.rb

# How Clearance / Hoptoad does it
module Clearance
  class << self
    attr_accessor :configuration
  end

  def self.configure
    self.configuration ||= Configuration.new
    yield(configuration)
  end

  class Configuration
    attr_accessor :mailer_sender

    def initialize
      @mailer_sender = 'donotreply@example.com'
    end
  end
end

# to use
Clearance.configure do |config|
  config.mailer_sender = 'donotreply@example.com'
end

# How Devise does it
module Devise
  mattr_accessor :mailer_sender # requires active support
  @@mailer_sender = 'donotreply@example.com'

  class << self
    def setup
      yield self
    end
  end
end

# to use
Devise.setup do |config|
  config.mailer_sender = 'donotreply@example.com'
end

# references: http://robots.thoughtbot.com/post/344833329/mygem-configure-block