Why a site goes into the supplemental index in Google ?

Some of the reasons why a site goes into the supplemental index of Google are:

1. Duplicate content: One of the main reasons why site goes into the supplemental index of Google .So try and make your content as unique as possible.

2. Same Title and Description in a page: Make sure you have separate title and description in each page.

3. Canonical Issues: Use 301 redirect to redirect your site from without www to www (e.g http://www.domain name .com/)

4. Lack of content: Build a site with good combination of text and images.

5. Lack of PR: Concentrate on link exchange and get as many quality backlinks as possible.

6. Orphan Pages: Pages that no one links to. So make sure you have clear navigation in your site.

7. Dynamic URLs: Pages with too many query strings in the URL. Search engines don't like them so always use static URL.

Follow these points and surely its going to help your site come out of the supplemental index of Google.

1. Eliminate the duplicate content issues on your pages. Page TITLE and Meta Description tags should be unique (and Meta Keywords, too, if you use it). Try to limit as much duplicate content as possible. It's okay to have a common header, footer, and site menu on your pages as long as your pages have enough other content to offset the common stuff. It's not okay for your red widget and blue widget product pages to have the same exact product description and page content, with the exception of the color.
   
   2. Improve the content on the supplemental pages. As mentioned in #1, pages with little or no content are strong candidates to land in the supplemental index. That's especially true if the amount of duplicate content (header, footer, etc.) is substantially more than the non-duplicate content.
   
   3. Improve your site architecture. If you have pages that are 4-5 levels deep (i.e., below or away from the home page), redo that structure so the pages aren't buried like that. Make it easier for the crawler to reach all your pages.
   
   4. Improve your internal linking. Have a site map that includes links to all your pages. This'll help make sure pages don't get orphaned. Be sure to link to the site map from your home page so the crawlers can find all those links easily.
   
   5. Fix your link profile. Who you link to and who links to you impacts the level of trust Google gives you, and less trust often equals more supplemental pages.
   
   6. Fix your URLs. Do you really need to pass 3-4 variables in the URL? Cut down on the parameters. Keep your URLs as simple as possible.
   
   7. Are you a theft victim? If someone has stolen your content, it's possible Google is ranking the stolen content on the other site instead of yours. Use a service like Copyscape to check for content theft. “Plagiarism Today” is another good resource to learn about stopping content theft, with specific instructions for contacting the offending party/parties.
   
   8. If all else fails, take drastic measures. Rename/save all the supplemental pages with a new URL and link to the "new" pages somewhat prominently on your site. The site map idea mentioned above would be a good start. And then do a 301 redirect from the old URLs to the new ones. (This is obviously a lot of work if you have a lot of pages in the supplemental index.)
   
   9. And finally, tell Google what's going on. When you're convinced you've exhausted all of these issues, use Google's Webmaster Central to submit your URLs directly into Google's brain.