For platforms such as Wordpress, you can choose plug-ins to add. For hosted solutions such as Blogger, you might have to edit the template, e.g. the design you like might not have a Description tag, so you have to add one.
I blogged about this some months ago:
http://www.netmagellan.com/seo/wordpress-seo-plugins/ but since then I have added other plug-ins for Wordpress. You have to think "traffic" more so than "SEO" so that you can find other plug-ins to help bring human traffic via RSS readers, directories etc.