If your page is well optimized, then your title element should contain the targeted keyword phrase(s) for that URL. And the keyword phrase(s) from the title element (and slight variations) “should” be the phrases most often used to find the page in the SERPs. By using all of the keywords from the title in the meta description, you should maximize the percentage of times that your meta description is shown as the Google snippet. Google will almost always show your meta description as the search snippet if the meta description contains ALL of the keywords from the user’s search phrase.
http://www.webmasterworld.com/google/4045085.htm