The following are common SEO mistakes which you should avoid.
Targeting the wrong keywords
Many people are guilty of making this mistake. People select keywords that are descriptive of their site but which average users may not search for. For example, if you have a site about relationships, the keyword phrase "relationship guide" may not work very well, even though it has the word "relationship". However, "dating advice" will be much more effective. Choosing the right keywords can build or break your SEO campaigns. Even if you are very creative, you can't think up by yourself all the great keywords. However, you can find many keyword suggestion tools on the internet which will help you find keywords that would be appropriate for your site.
Ignoring the title tag
Leaving title tags empty is a common SEO mistake. This is one of the vital places to insert the keyword, because it not only helps in optimization, but the text in the title tag appears in the search results as the page title.
A flash website with no html option
Flash could be attractive but not to users and search engines. If you have to have a flash-based site and you want to attract search engines, offer an html version. Search engines don't like flash because 'spiders' can't read flash content and so cannot index it.
Lack of maintenance and consistency
Many people think that once a site is optimized, it is done forever. If you want to succeed, you need to optimize your site and while keeping an eye on your competition, as well as changes in search engine ranking algorithms.
Focusing too much on meta tags
Many people think SEO is all about getting your meta description and keywords correct. Actually, meta tags are slowly becoming a thing of the past. Don't expect to rank well based on your meta keywords and descriptions.
Using images only for headings
Many people find an image to be more attractive than text for menus and headings. True, an image can make your site look good but in terms of SEO, images for menus and headings are a big mistake.
Many people don't realize how vital a good URL is. Absence of keywords is a common SEO mistake. Having keywords in your URL gives you more advantage over your competition.
There is a common deception that more backlinks are always better. As a result, many webmasters resort to newsgroup/forum spam and link farms, ultimately leading to penalization of their sites. Feel free to use backlinks for your site, but make sure they are quality backlinks.
Absence of keywords in the content
When you identify your keywords, adjust your content and insert the keywords where appropriate. You might want to highlight them or make them bold.