I recently looked back at some of the SEO reports we’ve done here at Conversion Counts and some SEO mistakes kept surfacing up. While a few mistakes are down to lack of knowledge which is totally acceptable, other mistakes are because there’s been an attempt to save where saving should not have been made, for example on quality hosting, dedicated IP address etc.
30 common SEO mistakes:
1. Not paying enough attention to content duplication issues
2. Having multiple domains all showing the same website version, not knowing that Google is smart and will simply disregard the all but one domain
3. Not paying a bit extra for quality dedicated hosting package
4. Not investing enough resources into creating quality content
5. Targeting too many keywords on page (or even worse, keyword stuffing)
6. Not targeting the right keywords due to lack of strategy
7. Using doorway pages which quickly disappear from the index
8. Creating content for the engines, not for the users
9. Using only images as the site navigation
10. When images are used, there’s little use of alt text or optimized image names
11. Using a non-crawlable JavaScript navigation
12. Not using metadata such as page title and description
13. Missing the connection between great design and SEO
14. Having a flash website intro (splash page)
15. Having a flash site all together 
16. Not maintaining the code which results in site errors
17. Submitting the site to 1000 junk directories
18. Counting on link exchange schemes as a link building strategy
19. Using blog comments and news groups to spread spam links
20. Considering (even for a second) answering that email which promised #1 position on Google for £99
21. Opting for a cheap shopping cart software which cannot be customized
22. Blocking important parts of the site using the robots.txt file not as it was meant to be used
23. No following important pages which were passing link juice before
24. Having multiple versions of the homepage
25. Allowing both non www and www versions to resolve without using a 301 redirect
26. Forgetting about site maps (HTML and XML)
27. Missing the connection between site analytics and great SEO
28. Checking search engine ranking every day (or couple of times a day)
29. Optimising the site for PageRank… why?
30. Hosting a blog away from the main site instead of hosting it on the site as a directory
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Other great resources:
SEOMoz Blog – 12 Easy Mistakes That Plague Newcomers to the SEO Field
Search Engine Land – SEO “Don’ts”: 20 Fatal Mistakes You Must Avoid To Succeed