The first step is to determine whether the review is accurate. If there really is a problem with your business, you should work to fix it, or you can bet that someone else will leave a similar negative review in the future.
As for improving your search results, your best bet is to try replacing the negative review with content that you control. For instance, you could set up a Facebook Fan Page for your business, or create a Twitter account. You could also solicit positive reviews from some of your regular clients.