If you have been using an SEO plugin for your Wordpress site, or you are looking for a new option, you should consider Wordpress SEO from Yoast.
I just stumbled on this gem of a plugin after using the popular All in One SEO Pack for years. Wordpress SEO blows this plugin out of the park. It's literally packed to the gills with up-to-date SEO features and customization capabilities. To get a sense of what this plugin can accomplish for you, check out the The Definitive Guide To Higher Rankings For WordPress Sites. Even if you don't make the switch, the article is worth a read as it goes over the current SEO best practices.