application.html.erb

<%= flash_messages %>

application_helper.rb

FLASH_KEYS = [:error,:notice]
def flash_messages
  return unless messages = flash.keys.select{|k| FLASH_KEYS.include?(k)}
  formatted_messages = messages.map do |type|
    content_tag :div, :class => type.to_s, :id => 'flash' do
      flash[type]
    end
  end
  formatted_messages.join
end