Michael Campbell of Dynamic Media has succinctly described the purpose and method in how to write a description meta tag that sells the click.  The operative idea here is “sells the click”, because most folks don’t realize that this is really the singular purpose of a description meta tag. It’s not about SEO, it’s about how the ranked page appears on the SERP and it’s the singular thing that determines whether or not the searcher is going to click the link, regardless of its SERP ranking.

Check out Michael’s video. You’ll see what I’m talking about: