Google has announced that it will no longer show the Supplemental Result label in the SERPs. For details see Google Supplemental Label Out, PageRank Next?.
Most of the content of this post is still relevant.
Earlier in the year, many very large website owners were concerned that most of the web pages in their websites were ending up in the Google Supplemental Results Index rather than in the main Google Index. Clearly if a keyword query can be answered from the main Index, then your entries in the Supplemental Results Index will never see the light of day.
The Google robots cover an awful lot of web pages and need to put them somewhere. Shimon Sandler explained why web pages may end up in Google Supplemental Results and what you may need to do to avoid this. Matt Cuts of Google discussed why their Big Daddy infrastructure modification had caused the uproar and suggested that things would improve as they fine-tuned Big Daddy.
In fact the answer to all this is very simple. You’ll find it in a comment from Matt Cuts of Google in his blog:
Higher quality posts are the trick.
In general, the best way I know of to move sites from more supplemental to normal is to get high-quality links (don’t bother to get low-quality links just for links’ sake).
So if you’re a culprit, stop creating all those spammy web pages in the hope that quantity of links will in some way compensate for lack of quality in those links. That’s not the way to get out of the Google Supplemental Results index.
Tip of the hat to SEO Scoop, a blog worth watching.