Finally Google has shown how it picks the snippets. This is a gem worth keeping an eye one. Just imagine the possibilities if you could control the snippets that google shows for your website. (No I am not affiliated to that site). Thank You
They don't always use the snippet in your meta description tag. It depends on the search term. You should try to create a snippet that matches your main keyword and/or page title. Then they are more likely to use it.
Google's Matt Cutts did a video blog of this exact topic two weeks ago: http://www.mattcutts.com/blog/video-anatomy-of-a-search-snippet/
Google simply takes what it needs, there are many things that it pulls from: 1. Title Tag (H1) 2. H2's - H6's 3. Meta Description 4. Page Content 5. Keywords in URL These are just a few things Google will pull from...there are many other factors
Use all keywords of title tag in description tag & limit character length upto 150 characters including spaces......