tedster - 7:46 pm on Dec 18, 2007 (gmt 0)

There are some key factors I know of for keeping URLs out of the Supplemental Index:

1. Enough link juice circulating to the page - this is a big factor
2. Unique Title element - keep it relatively short and vary more than just stop words
3. Unique Meta description - and make it of some length (not just a couple words) but under 160 total characters
4. Accidental duplicate URLs

Duplicate urls for the same content can make problems - and sometimes this kind of duplication is not intentionally created. Things like canonical urls, switching the order of query string parameters, not handling custom error pages with a true 404 status, placing tracking data in a url ... the list of potential technical oversights really can get huge.

So I'd say make sure there are relatively short click paths to your inner pages, either from the Home Page or other pages that have solid backlinks. Ensure that you have unique and page-specific titles and meta descriptions. And get intense about eliminating all kinds of duplicate URL issues.

But the biggest is having some strong link juice - good PR. As backlinks grow, then you have more PR to cireculate. Adding new pages without attracting more link juice can tend to divide the available PageRank into smaller bits. This doesn't mean don't add new content, it means be sure to market what you create.