There are a lot of plugins that can be helpful to your SEO campaign and Wordpress blog. I did a search on Google and came up with this site, Top 5 SEO Plugins for Wordpress . Hopefully this helps.
I think Platinum SEO or All in One SEO are great, but make sure to use a cache program, 301 redirect, and permalink plugins as well. They'll help you setup your link structures, web speed, and more.
Plus, do you have a sitemap and PCRobots.txt?