I recently received a question from one of my friends regarding the use of keywords and descriptions within meta tags on a website to improve ranking of their site. They asked if they could increase the frequency of how often those tags were shown so they could improve the ranking of their site. My response was as follows:
The keywords aren't pushed out anywhere or anything, so there is no frequency you can adjust. The metadata is embedded as part of each page. When Google, Yahoo, or any other search engine crawls your site they read the content of your webpage as well as that additional meta information.
Honestly, the use of the meta keywords and description tags have changed significantly. They used to be critical in ensuring a top-spot in the top-ranked search engines, but all of that has changed. Google is more focused on making sure that truly good content gets to it's users. They do this through a number of methods, but the most important thing I can recommend is to make sure you have good content.
If you are interested check out the following link: http://www.seomoz.org/
As you'll notice in the article, a number of the things are simply based around content: use keywords in the title, around images, in links, in body text, etc. Other critical items are somewhat outside of your control such as having links from highly-ranked sites within your category link to you, having good descriptions in the link-ins, etc.
That being said, just focus on having good quality, focused content and over time the search engines will pick you up.