If you like Atom more than RSS use the atom_feed_helper.

Here's the template, modify it to fit your needs. I know there are plugins and other ways of doing this, but I hate code that gets too abstract:

<?xml version="1.0"?>
<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
  <channel>
    <atom:link href="http://xxxxxxx" rel="self" type="application/rss+xml" />
    <title>Code Snippets - Aktagon</title>
    <link>http://snippets.aktagon.com/</link>
    <description>Share your code with the world. Allow others to review and comment.</description>
    <language>en-us</language>
    <pubDate><%= @snippets[0].created_at.rfc822 %></pubDate>
    <lastBuildDate><%= @snippets[0].created_at.rfc822 %></lastBuildDate>
    <docs>http://blogs.law.harvard.edu/tech/rss</docs>
    <generator>Aktagon Snippets</generator>
 <% for snippet in @snippets %>
    <item>
      <title><![CDATA[<%= snippet.title %>]]></title>
      <link><%= snippet_url(snippet) %></link>
      <description><![CDATA[<%= snippet.rendered_body %>]]></description>
      <pubDate><%= @snippets[0].created_at.rfc822 %></pubDate>
      <guid><%= snippet_url(snippet) %></guid>
      <% for tag in snippet.tags%>
        <category domain="http://snippets.aktagon.com/snippets"><![CDATA[<%= tag.name %>]]></category>
      <% end%>
    </item>
<% end %>
  </channel>
</rss>

Remember to serve the feed with the correct HTTP headers.

It also helps to have an auto-discovery tag inside the head tag:

<link rel="alternate" type="application/rss+xml" title="RSS feed" href="http://<%= request.host %>/rss/" />