My favorite reasons that pages go SI:

1. Exactly duplicated content
2. Over the top content/structure similarities
3. Pages with little or no content
4. Orphaned web pages. Pages that no one links to, including internal navigation
5. Poor website navigation
6. Pages due to canonicalization problems
7. Not enough quality back links
8. Long URLs, especially with long parameters, starting with a question mark (?) and being separated with an ampersand (&) and are not rewritten
9. Suspicious pages for spam-indexing, like non-unique and irrelevant to page content Meta tags, or linking to bad neighborhoods, etc.
10. Using duplicate description tags and title attribute across your website

So simply not doing these things will help keep you out.

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