Optimizing a website for the search engines is in two logical parts; on-page optimization and off-page optimization.

On-page is everything that is done on the website’s pages to improve search engine rankings and off-page is everything else, like obtaining inbound links for example.

As a web designer you can control on-page factors but not off-page factors.

Very few designers know even the basics of search engine optimization and if you were to study it for a while your clients will benefit substantially and hence so will your business

Start with this excellent tutorial http://www.seo-blog.com/tutorial.php and by the end you will be providing your clients with a lot more!
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