As with the meta keywords tag, your page's meta description tag doesn't help your search engine rankings much, but it does allow you some control over how the page is described in search engine results. Many search engines (though not Google), will use what you have put in the meta description tag on the search results page.
Presumably, you have written your post following the old advice to tell 'em what you'll tell 'em, tell 'em, then tell 'em what you told 'em. Using this format, the first part of your post will give an excellent excerpt for the search engines to display and persuade people to click through to your site.
The MetaDescription plugin is designed to automate this process for you. It will read your post or page, generate a meta description tag from the first part of the content, then inject it into your page header. The plugin will also generate a meta description tag for your home page. This defaults to your blog's tagline, but is easily configured to anything you want.
You can download MetaDescription from the plugins page.