Two of the plugins we have used for fine tuning sites for Search Engine Optimization are All in One SEO and WordPress SEO by Yoast. When we set up our latest sites we decided to use WordPress SEO due to it's tighter integration with the latest Genesis theme framework we are using.
After researching the best practices from several sources and Joost de Valk himself (Yoast), we decided to turn off automatic generation of Meta Descriptions. This is not to say that Meta Descriptions are not important, because they are very important, but if you are not going to hand craft the description yourself, you are better off letting Google (or the search engine crawling your site), determine the description themselves.
We encourage you to check out the plugins official site for a complete explanation and the Definitive Guide for the WordPress SEO.
For a few years now, we have been reading that keywords are not really worth the effort. The major search engines don't really rely on these so there seems to be no point in taking time to enter them. By default, the keyword field is turned off for all posts and pages, but you are welcome to turn in back on in the WordPress SEO plugin General settings. For more on this subject be sure to read Meta Keywords: why I don’t use them.