First, I would recommend talking to the person who left the bad review if that is an option. If it isn't, then I would think about the comments for a day or so before responding. Draft a general response that accurately represents your business philosophy without trashing the person who left the negative response. If their review was unwarranted, you don't want to bolster their credibility by taking stabs at their character or otherwise sinking to their level. Be honest about your practices and if you made a mistake, own up to it and offer resolution. Don't give too much detail, just say something that conveys the message that your intent is to provide quality services and that if you have failed in some way, you are more than willing to discuss options to resolve the matter. Maintain your integrity and you will do much to preserve your reputation even when someone else is trying to tear it down. Hope that helps.