Well, to answer this question I can write a book, but let's cut the long story short and get acquainted with the significant factors that can save your pages from being supplemented.
Life savior of your pages is Backlinks. If you have average quality or even good quality less content on your site but your site has several backlinks, you are saved from supplement index.
Even, if it has already trapped into supplement index, backlinks can do wonders for the site in digging out it from the entanglement of supplement index. Lousy content with good number of backlinks also works. I think backlinks are Google's favorite. Be cautious, backlinks don't any link from any website, they should be relevant and good quality links because they are the trust building factors.
Content Optimization also plays a significant role in avoiding pages from getting supplemented, but sometimes your unique, good quality article or page may too become a part of supplement index due to the use of similiar Website Templates. I think this is why directories are being hit hard by this, too little unique content on one page compared to the rest of the page. Also, unique content in your article with appreciably similar titles may also become the reason of falling into Google's pit of supplement pages because Copyscape may think content under the same title name will be duplicate.
Other some other important factors that count are Meta data, RSS feed, Outgoing Links and Duplicate URLs. Although meta data doesn't hold much importance in the eyes of Google, but distinct meta titles and descriptions can help Google understand that the pages are different from each other. While using RSS feed make sure you are using the no follow tag in your feeds or just use the robots.txt to prevent indexing of the feeds.
Overall TrustRank, age, etc are also accountable for supplement index but above mentioned points hold more weightage then others. So, keep in mind all the above said things and sparingly do the Optimization of Your Website, to save it from any havoc.
This article is published by Search Engine Optimization Company Dimension India Seo Consultants.