Great article by Joe Lewis:
http://www.webpronews.com/topnews/topnews/wpn-60-20061221EscapingGooglesSupplementalDeadZone.html
Basic Reasons:
• Duplicate content. This is often the main reason a page ends up in the supplemental index.
• Too many variables (parameters) in the URL. Google mentions this on the “help” page linked up above.
• Poor overall link profile. Matt Cutts specifically mentioned earlier this year that the Bigdaddy software upgrade would result in more supplemental results for “sites where our algorithms had very low trust in the inlinks or the outlinks of that site. Examples that might cause that include excessive reciprocal links, linking to spammy neighborhoods on the web, or link buying/selling.”
• The page is buried. Orphaned pages are candidates to go supplemental. These are pages which can only be reached by a deep crawl of your site’s internal links, or pages which can’t be reached at all.
Joe Whyte offers these tips:
• Remove all duplicate content and write your own copy for your pages.
• If you don’t have content or enough content on these pages that are supplemental then you need to create more.
Once you make these changes your pages will be revisited and re-cached. You can also create a Google sitemap in order to help Google crawl more pages of your site.
The Google Success blog adds:
• You may need to rewrite your PHP code for e-commerce websites using mod-rewrite to simplify the cryptic URLs and also add unique meta tags to each page.