The principle is: Don’t just respond to the review. Resolve the issue.
1. Own up to your mistake readily if the complaint is true.
Say sorry immediately.
2. Offer next steps for the complaining customer.
Suggest whom should he contact, or give them a special coupon, or something.
3. Explain what you will be doing to make sure it doesn't happen again.
Quickly say why things went wrong, and how you are making sure things go right as promised next time.
4. Humanize your brand.
Post a video on Youtube, with your response (see above). People warm up on seeing an actual person behind the business.
5. Use other online channels to provide support.
Twitter is a very effective customer support channel.
6. Ignore the rants
Just focus on the complaints where you can actually get some details, and do something about it. People will notice your pro-activeness with customer support anyway.