Ok, I’ve run into this problem a few times and keep forgetting what the solution is, so here’s a good reference post for myself and the rest of you wondering.
Let’s say you have in your WordPress install 5 or more categories, but to the general public, there’s only two categories you want to show (such as the case with http://www.midmarketinnovators.com/). In this blog there’s multipule categories, some of them for a public post / Q&A section and the other for the author blog posts and still, some more for the sidebar featured products section. Well, to the general public, they don’t need to see the Featured Products or the Q&A section in the general feed, but rather just the blog posts, from the various authors partisipating. So let’s say they post to the category ID’s 22 & 23, well in your link tag in the header place the following tag.
<link rel="alternate" type="application/rss+xml"
title="<?php bloginfo('name'); ?> RSS Feed"
href="<?php bloginfo('rss2_url'); ?>?cat=22,23" />
Which outputs in html as
<link rel="alternate" type="application/rss+xml"
title="Mid Market Innovators RSS Feed"
href="http://www.midmarketinnovators.com/feed/?cat=22,23" />
In the case of Mid Market Innovators, the feed was placed into a feedburner plugin, which redirects the http://www.midmarketinnovators.com/feed to the feedburner link. But, this should still work if you’re not using feedburner. If you run into any troubles, post a comment.