xml.instruct!
xml.urlset "xmlns" => "http://www.google.com/schemas/sitemap/0.84" do
  xml.url do
    xml.loc         "http://www.YOUR_SITE_URL.com/"
    xml.lastmod     w3c_date @posts.first.updated_at
    xml.changefreq  "daily"
  end

  @posts.each do |post|
    xml.url do
      xml.loc         url_for :only_path => false, :controller => 'archive', :action => 'show', :id => post.id
      xml.lastmod     w3c_date post.updated_at
      xml.changefreq  "never"
    end
  end
end