Your own customers' testimonials can be a powerful marketing tool, but asking them for a recommendation can backfire if you're not careful. Here's how to turn customers into evangelists for your brand, according to Michael V. Pantalon, a clinical psychologist and researcher at Yale School of Medicine and author of Instant Influence: How to Get Anyone to Do Anything--Fast.

1. Ask, don't order.

"Jumping in cold with a request that customers tell the world how much they like your products or services is a huge mistake," says Pantalon. Your request to "Like us on Facebook!" takes too much for granted and can come off as an order. Pantalon suggests a softer approach, such as "I'd like to ask if you'd consider speaking with me about getting word out about the brand."

2. Emphasize their autonomy.

If customers are willing to speak well of you, encourage them to express themselves however they like, advises Pantalon, while underscoring that how they respond is totally their decision. Don't try to tell them what to say or what words to use, he says: "They're not there to transcribe, and it's important you reinforce their control."

3. Be positive.

Avoid negatives, such as "Why don't you post something nice about us!" The negative "don't" adds unnecessary pressure to the request. Instead, focus on how your customers have benefited as a reason for their praise. For example, ask people how their purchase or your service was memorable or helpful. "Consumers know that their positive recommendation will be a plus for you, but they often need to be reminded that they've gotten something good from your business," says Pantalon. "Value is a two-way street."