1. Using the wrong keywords. This is such an easy mistake to make. One of the biggest problems is that people tend to go after the search terms that get the most searches, but that isn't always the smartest way to go. The keywords that get the most searches almost always have the most competition and can put you at a disadvantage when starting out. It's okay to use keywords with fewer searches if they fit in with your overall SEO plan.
2. Tag confusion. A few years ago you could get away with stuffing your meta tags with all of your keywords, and that would be enough to move you higher up in the search engines. Today, a lot of the bigger search engines virtually ignore your meta tags when determining where you will place in the results. However, that doesn't mean that other tags don't matter. You can still use meta tags, but you don't have to stuff them with keywords, instead, use them to write a good description of your site and fill in other useful meta information. Heading tags, alt tags and tags used for emphasis can give your keywords and images extra weight, so use them wisely.
3. Having a site that uses Flash only. Flash is a wonderful tool when used properly, but the spiders sent by the search engines can't read it. Not everybody likes a Flash-only website, but if you have good reasons for using Flash, be sure to include an HTML version of your website so the search engines have something to rank your site with.
4. Java Script. Similar to Flash, Java Script can't be read by the search engines. It's easy to use various Java Script applications on your website, but be careful with where and how you use them. One of the more common SEO mistakes is to use Java for menus on a website. That's one of the worst ways to use Java Script because you will lose all of the SEO benefits that come with internally linking your pages. In other words, if you must use Java Script, don't use it for your menus.
5. Relying black hat SEO techniques. Yes, black hat SEO can and does work, but only for a while; eventually the big search engines figure out the black hat methods and then they change their algorithms to make those methods ineffective. When this happens, your page can move down in the results instantly. In the long run, black hat SEO takes more work, so do your best to play by the rules.
Avoiding these common SEO mistakes will help you place higher in the search engine results.