The main reason a webpage or a website gets into the Supplemental Index is the lack of internal links. When the Google spiders crawl deep into your website, they evaluate the web pages they find with those that are in their index. If the web pages exist in the Index that the Google Spider did not find during its deep crawl, then the Google Spider considers those pages as inaccessible through any internal link. This leads your web pages to get into the Supplemental Index.
The following is a list of points that you need to keep in mind to get your webpage or website out of the Supplemental Index:
1. Webpage not updated
If the web pages of your website have not been updated for more than 1 year, then there is a very high chance that those web pages would be placed in the Supplemental Index.
Tip: Make a few small modifications on each webpage of your website by tweaking the On Page Data or even the content of your webpage.
2. Missing a direct link from the index page
The webpage in question may no longer be getting a direct link from the index page or home page. Such pages would also land up in the Supplemental Index.
Tip: Re-establish the link between the home page and the page that has lost its link with the home page or any of the main category pages of your website.
3. Domain Server
Ensure that you have control of every domain that is associated with the IP address. If you are using a virtual server, then you must own your own server. Sharing an IP address across various domains is not good.
4. Directory, Article and Press Release Submission
Submit your site to those directories that are relevant to your category or industry. Also do Article and Press Release submissions by placing links back to your site and also post them on major news feed sites.
5. Link Campaign
Start a link campaign by getting quality links pointing back to your home page and also the internal pages of your website.