There are no widely accepted standards regarding RSS or XML buttons. The most common is the incredibly ugly bright orange XML button introduced by Userland, which usually points with a standard href directly to the feed. This tends not to be particularly useful to the end user - if the feed is served as the legacy
text/xml then the feed is shown as a DOM tree, and if you are using application/rss+xml or application/atom+xml then unless you write an XSL stylesheet most browsers will offer a download prompt.
The best way is to complement a direct link with a link to the RSS/Atom file within the
head of the HTML page, like this: <link rel="alternate" type="application/rss+xml" href="/filename.rss" title="The title of my feed">
This method is less obtrusive and is supported by most auto-discovery systems out there.