Targetting the wrong keywords
This is probably the mistake that is made the most, even be experienced web developers. You might find the right keywords that describe your web page, but the users use other keywords. You must think like the user does, and see what keywords are used more than others. There are tools to track down the big searches on most search engines, so it is advised that you use those!
Ignoring the Title tag
The title-tag is a very important meta tag, and it’s very often left empty, or sometimes a website has the same title for all it’s pages. Remember that the text in the title tag is shown on the search results. Also it’s a good place to add some keywords. Don’t forget it!
Flash website without a html alternative
Flash is very cool and it makes your website stand out for the users, but not so much for search engines. Always provide an html-alternative, so the search engines can crawl your webpage and show your website in their results.
Concentrating too much on meta tags
Meta tags used to be very important but nowadays are becoming a thing of the past. It is important to focus on meta tags, but it isn’t what’s going to get you up in the ranking. When you hire external SEO consultants, they often over-focus on meta tags because this is something they can easily do and charge you for. Beware!
Using only Images for Headings
Images for headings look nice, and users may like it, but once again search engines can’t read it. Use -tags for headings, search engines love them. And with a little bit CSS they will look just as nice as the images!
It is paramount that you have a good URL. Dynamic page names are very common, and there’s a lot of pages in there that have no keywords in there. Of course some of these pages rank very high but they could rank so much higher if their URL would at least have one keyword in it. Put keywords in the domain name, the folders and the page itself. This will boost your ranking!
It’s important to have backlinks, but make sure the backlinks are descent. You rather want five backlinks that have something to do with your subject, than 50 backlinks all from linkfarms or newsgroup spam. This might get your site banned from search engine results.
Lack of keywords in the content
Once you focus on your keywords, modify your content and put the keywords wherever it makes sense. It is even better to make them bold or highlight them.