My little experience of doing SEO has taught me a few things. I have compiled a small list of common SEO mistakes done by me and also by others who are starting out on doing SEO by themselves for their websites.

1. Building high amount of links to your website quickly.
2. Use of high Keyword density in your article.
3. Duplicate content across your website.
4. Repeatition of title across your website.
5. Use of automated tool or software for spamming your links.
6. Not using the meta keyword and description tag
7. Linking from a non related website.
8. Not using sitemap
9. Using spinned content which replaces the article by synonyms, [There is a small controversy regarding this. SEO specialists say that use of automated article re-writers which makes the article unique by replacing words with their synonyms are bad for your website].
10. Bad internal linking structure for your website.
11. Changing your website title frequently.
12. Bad url structure for your websites.
13. Pinging your blog posts too often and at a quick interval.

These are some mistakes which are often neglected by webmasters and these often add up to you being banned from search engines