If your title tag is considered the headline then think of your description tag as the ad copy below the headline. A good description tag will compel the reader to visit your website.
Google cuts off the displayed description tag at about 150 characters, but may index up to 185 characters. Other engines may go up to 250 characters, but will still only display about 150. Therefore, try to say what you've got to say in 150 characters or less, but feel free to use up to 180, and even up to 250 if you're not concerned with the entire thing getting indexed by all the search engines.
Your description tag is not only sales copy for people to read, deciding to click your ad or not, but it's also food for the hungry search engine spiders. There's probably no stronger or more important meta tag than the description tag.
However, one mistake I see a lot is people trying to stuff too much information into their meta description tag, then reusing that meta description in multiple places on their website. Don't describe your whole business, just describe the page in question.
For example, let's say you're a contractor, and you provide plumbing, remodeling, and electrical work. You would want to have a completely different meta description tag on each of your service pages. Don't make the mistake of using one description for all your pages.
Use your primary keyphrase in the beginning of your description tag too. This gives a higher keyword prominence percentage to the search engines and your site will rank higher. Don't OVERuse your keyphrase though, or it can be considered webspam. Higher then zero keyword prominence is good, but higher still is not always better. Use natural sounding language, proper grammar, and try to peak interest in the subject. Remember, you want the reader to click your ad, not just to be found. how many times have you skipped over a search result because the two lines of text you see in the SERPS were just gibberish.
Finally, if you're a local business, i would encourage you to put your address, including your zip code in your description meta tag. I usually put it after the 150th character, but complete it before the 180th. the reason for this is that there's strong evidence to suggest that this will help your local search results. as more and more search engines are trying to determine what their visitors want, local search matters now more than ever.