example.rb

# Create the plugin
./script/generate plugin flash_helper

# init.rb
require File.dirname(__FILE__) + '/lib/flash_helper'
ActionView::Base.send(:include, FlashHelper)

# flash_helper.rb
module FlashHelper
  def display_flash(flash)
    ret = []
    flash.each do |key, value|
      ret << content_tag(:div, value, :id => "flash_#{key}")
    end
    return ret.join("\n")
  end
end

# application.html.erb
<%= display_flash(flash) %>