I have mentioned this in an article I once wrote about blogging (http://www.webmastervault.com/articles/advanced_blogging/) but I wanted to touch on it again because it is very important. Google and Yahoo have essentially decided on a standardized way of presenting and reading sitemaps. I’m not just talking about a hand made site map that you put together, but an actual XML site map that basically looks like nothing more than a bunch of code with urls. These sitemaps are read by google and yahoo and they are used to index you site. They tell these SE’s when you updated your site last and what you added. If you look at them, and you are not proficient in XML, they are going to look like an alien language, but it doesn’t matter the SE’s can read them so that is all that matters.

With this plugin your blog will automatically update the sitemap when you add or delete posts and it will ping the SE’s to let them know the site has been updated. http://www.dagondesign.com/articles/sitemap-generator-plugin-for-wordpress/
This is the plugin I use and like. It is easy to set-up and use and has some really nice features in it.

http://www.sitemaps.org/
This is a great site that gives you information on what a site map is, what it is used for and how to use it on not just your blogs, but your regular sites as well.