1. Do not link to non-trusted, low quality websites. If its absolutely necessary, use the nofollow tags.
2. Do not over emphasize keywords on articles. Keyword density is an old SEO metric. Get over it.
3. Do not over load pages with Ads/Scripts/Template scripts that will increase page load time.
4. Do not copy content from other websites.
5. Do not leave an unnatural number of broken links / pages on your website.
6. Do not use hidden keywords in pages.
7. Do not use cloaking.
8. Do not purchase links (at least from non-trusted sources).
9. Do not entertain link exchange campaigns.
10. Do not leave URLs to be dynamic.
12. Do not get into link building strategies/campaigns that will get you links too quickly within a short period of time, say 100 links in 1 months.
13. Do not squeeze in every bit of keyword you can possibly imagine to page titles/meta tags/meta keywords. Just don’t.
14. Make sure your header tags are working properly, 301 redirects etc not giving wrong signals to search engine bots.
15. Do not use automatic publishing scripts/plugins to build content. Saves a lot of time, but you’re sure to get a surprise from Google one fine day. Don’t complain then, act now.
16. Do not make websites just for the sake of selling ads/affiliate links or AdSense. With the new updates Google discounts “zero content” websites heavily from search results.
17. Do not thin out pages on your website with more auto generated template content and less real content. There are pages where you have to sieve through ads and other stuff to find the real content. Don’t do it.
18. Do not host websites on weak servers that cannot deal with traffic load. Last thing you need is frequent down times. Getting on to a dedicated server that can deal with any traffic and loads up pages in seconds is the best choice.
19. Do not host websites on IPs that are “bad neighborhood”. You might get blacklisted for “reasons not yours”. Check for possible IP issues before getting on shared hosting or buy unique IPs.
20. Do not write for machines. Write for people, the way they like it. Real popularity is more important than search engine friendly-ness. And Google is learning how to figure that out.