I had faced this problem for quite sometime now, and I have come to the conclusion that it is all about links. If your content rich and unique page is linked from other sites, then very unlikely that it will go into supplemental index.

Here are some findings:

1. Even completely unique page with content will go into supplemental if your site is not trusted. I don’t know how and what criteria they use, but I have seen it. Supplemental index does not always mean duplicate content.

2. Internal links can help, acts as a backlink, so make good use of it to get out of supplemental. Try linking from your homepage for better results.

3. Pointing good quality links to your mainpage will help gain trust with Google, and thus you will have less pages going into supplemental.

4. Point links (preferly good quality links) to the page that went supplemental. It will go out of supplemental soon.

5. Text rich and unique pages will be less prone to supplemental. Kind of contradictory to the first point, but it is true.

6. Pages in the Supplemental results may be temporarily when Google does some sort of dance or shuffle. It will be out after a while (one or two weeks time?). Some however will be there for a long time if nothing is done about it.

7. Updating your page often can help. Having more text on your page can also help.

8. Sites with a lot of pages will tend to have some of them in supplemental. Perhaps it is due to internal linking problems or that Google doesn’t like too many pages of a same site clouding it’s search results, but this is what I have discovered.

Well, this is what I have. I will update this page if I can anymore information on this supplemental hell thing. Hey, if you have any comments or if you think what I say is wrong, please leave a comment. Thanks.