sitemap.xml.erb

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <url>
    <loc>http://robots.thoughtbot.com</loc>
    <priority>1.0</priority>
    <changefreq>daily</changefreq>
  </url>
<% sitemap_articles.each do |page| %>
  <url>
    <loc>http://robots.thoughtbot.com/<%= page.destination_path.gsub("/index.html", "")%></loc>
    <priority>0.8</priority>
  </url>
<% end %>
<% sitemap_tags.each do |page| %>
  <url>
    <loc>http://robots.thoughtbot.com/<%= page.destination_path.gsub("/index.html", "")%></loc>
    <priority>0.2</priority>
    <changefreq>weekly</changefreq>
  </url>
<% end %>
</urlset>

sitemap_helpers.rb

module SitemapHelpers
  def sitemap_articles
    sitemap.resources.select do |page|
      page.path.start_with?("posts/")
    end
  end

  def sitemap_tags
    sitemap.resources.select do |page|
      page.path.start_with?("tags/")
    end
  end
end

Thanks!