In this article we cover some of the worst SEO mistakes. Some of these will cause the search engines to ignore you; others will cause potential visitors to ignore you; still others may make your site disappear from the search engine results pages entirely. We'll go over each practice and explain why you shouldn't do it. We'll even tell you what you should be doing.
Flash
If you're making a new website and thinking of using Flash - stop. Though Adobe made Flash crawlable and shared technology with Google and Yahoo (leaving out Microsoft), Flash is still a bad choice.
If you don't show much content to the search engines, you will have to invest more in links. Not only does a Flash site cost more to make, but you'll need more money to optimize it. If there is no real reason to use Flash, don't. If you want cool movies and features, consider embedding Flash videos and using cool CSS styles.
No Use of Title Tags
"The most powerful HTML tag you have at your disposal" - Ross Jones, Search Engine Ranking Factors. Put keywords you target the page for into the tag. This is the part that shows up in search results as a big blue link, so make it count.
Another mistake webmasters make is to put something like "Welcome To...." in the title tag. Aaron Wall calls this "Welcome to low rankings."
Title Tag Duplication
Many designers create a site design without much regard for SEO, using one title tag for an entire website. Make sure to vary the title tags on your pages.
Messed up Robots.txt
It's a good idea to have a file to keep your error log clean of robot.txt requests, but if you can mess up a lot if you use it incorrectly. Here's how it looks, assuming you don't want to block ANY pages:
User-agent: *
Disallow:
Not Using Keywords in Internal Links and Navigation
Your internal onsite links have as much SEO weight as some outside links. If your home page has link authority, then links from your home page to internal pages pass pagerank in the same manner as outside sites pass pagerank to your home page.
By linking from your most powerful pages to internal pages with keyword-rich links, you're doing exactly the same thing as getting keyword-rich inbound links from high PR pages.
To make the most use of link power, make sure that your navigation consists of keyword-rich, targeted descriptions. Do the same with keywords scattered within content. Don't worry about your footer; Google doesn't put much weight on those.
Search Optimization 
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