The description meta tag is intended to provide the text search engines should use along with your title when displaying search results.  Of course, as with most things in SEO this does not always happen.  Many search engines, Google included, will sometimes create their own description that they feel better match the search query.

<meta name="description" content="your unique page description" />

Descriptions should be targeted at the surfer to entice them to your listing because most search engines give low ( if any ) weight to the description tag for ranking purposes.  The primary value of the description tag is that it allows you to partially control how your listings display in results.  If you provide compelling, unique, and accurate descriptions you will attract more search visitors.

An optimized description meta tag should follow these simple guideleines.

1. Unique description tag for every page
2. Use your primary keyword phrase once in the description
3. Accurately describe the page to reduce bounce rate

Unlike in the title tag there really are no “advanced SEO” tips for the meta description.  Search engines, with the exception of Yahoo, place low to no ranking value on the description meta tag.  Instead spend a few minutes creating a unique description that will appeal to searchers and then move on to something that will provide greater SEO returns.

The only time I would suggest revisiting the description tag is if you’re receiving significantly lower search engine traffic than your search position should be providing.  In that case, some optimization of the description may result in a higher click through rate.

Again, I wouldn’t consider this unless I was in a top 5 position.  Before that any time spent optimizing the description could be better spent building links or developing content to increase your rankings.