Via SearchEngineLand.com, here is a Google blog post about how to write a good META tag.

Improve snippets with a meta description makeover
Thursday, September 27, 2007 at 6:46 PM
Written by Raj Krishnan, Snippets Team

The quality of your snippet — the short text preview we display for each web result — can have a direct impact on the chances of your site being clicked (i.e. the amount of traffic Google sends your way). We use a number of strategies for selecting snippets, and you can control one of them by writing an informative meta description for each URL.