We use and recommend the WordPress SEO plugin by Yoast.

We have simply found this to be the best SEO Plugin on the market today. It is provided to our customers at no cost.

You can visit the plugin site for all the details.
http://yoast.com/wordpress/seo/

There is also an excellent tutorial to help you better understand in intricacies of SEO optimization for your blog.
http://yoast.com/articles/wordpress-seo/

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