One of the most important secrets in dealing with negative social media feedback is to reduce the need for it in the first place by making sure your customers know how to reach you, as directly and quickly as possible.
Customers need every possible opportunity to tell you to your face (or on the phone, or to your direct email inbox, or via live chat, or real-time video) how they're feeling. And they need to be able to see that you care, are listening, will consider their opinion.
The principle of the unzipped fly
Here's a colorful way to think about it: If your friend saw you had your fly undone, or spinach between your front teeth, would he tweet about it?
Of course not. He'd quietly tell you. (And if nobody tells you all day when you're fly's unzipped, it's proof positive that you have no friends!)
This is what I call "the principle of the unzipped fly," and it's very simple and powerful. Treat your customers like your friends-friends you want to hear from, day or night-and there's a good chance they'll return the favor. In other words: Why should customers address issues to you indirectly via Twitter or their blogs when they can use email, the phone, or a feedback form on your website and know that it will be answered--immediately? (Have ''chime in'' forms and "click here for an immediate response" options everywhere on your site; this is like building escape valves for steam into your machinery.
Have an after hours phone number-answered by someone who can actually help, rather than an answering service (unless that answering service is able to page said person-who-can-help). Answer your own email after hours. Have someone read those web queries and comments after hours, or at least the next morning--as opposed to "we'll get around to that, oh, how about never?" which is the reality at far too many businesses.
In a sentence: Be there when the customer needs you. Especially, be there when the customer first starts to feel misunderstood-before things get out of hand and onto the social media airwaves
Because the only other way to avoid getting complaints publicly, via social media is to never make a mistake. Ever.
Good luck with that one.