"Nine out of 10 consumers say word of mouth is the most important factor influencing what they'll buy," says Andy Sernowitz, CEO of the Word of Mouth Marketing Association.

Companies can protect themselves from harm. Obviously, the most important thing is to make sure products or services are rigorously tested and attain the highest quality possible before they hit the market. Customers also need to have a positive buying experience. But that's not the whole story:

Don't rely on momentum to sustain sales of inferior products. Sernowitz says many businesses still think they will milk a ho-hum item for years as long as they can still advertise it in a positive light. But nowadays, "consumers know whether [a product] is good or bad within 24 hours of it hitting the store shelf," he notes. Sernowitz advises companies to conduct thorough reviews of existing products, and those under development, making fixes rapidly or weeding out the duds very early.

First-time customers need special attention. They're the ones most likely to take the time to tell others about their buying experience. Whenever possible, make an effort to find out who among your customers are rookies, and give them the red-carpet treatment: personalized attention, token gifts, offers to participate in focus groups, special discounts on other products, etc.

An active online presence for your firm is vital to any attempt to guide discussions about things you sell. Consumers with interest in your products or services will naturally come calling, and if there's a blog or discussion room provided for them on the site, then they'll be tempted to chime in. This may give your company an early heads-up on complaints before they spread like wildfire around the Web.

Free online services can keep tabs on mentions of your business to let you know as soon as possible when chatter begins. Google Alerts will shoot your firm e-mails anytime your company's name pops up on a Web site or in a blog, while Technorati offers a search function to do the same thing.

Reply promptly, clearly and honestly to online comments or criticisms with posts either to the blogger or other sources of the opinion, or in a release on your firm's Web site, or both. If the comment identifies a real problem, address it by explaining how it will be fixed.

What's more, firms can turn word of mouth to their advantage by tapping viral or buzz marketing channels to build sales and bolster their image. New Line Cinema did so as it spent more than $300 million producing The Lord of the Rings trilogy. By engaging potential viewers online—for example, by giving them a forum to chat about the stories and offering them preview clips—New Line Cinema generated huge box-office interest. Footwear maker New Balance did the same earlier this year for a new line of athletic shoes.