When it comes to selling, referrals are the Holy Grail. It's much easier to make a sale when the sales process starts with a customer recommending you to a friend or colleague.

Ideally, you want to turn your best customers into a "volunteer sales force," according to Rob Fuggetta, author of the newly published book Brand Advocates: Turning Enthusiastic Customers Into a Powerful Marketing Force.

When I recently spoke with Fuggetta (who also heads the firm Zuberance) about referral selling, he gave me the following five tips:

1. Identify your best customers.

First, find out which of your existing customers are likely to join your "volunteer sales force." Use your website, blog, newsletter, or other customer touch points to ask the following question:

On a scale of 0 to 10, how likely are you to recommend our company or product?

Any customer who responds with a "9″ or a "10″ is a candidate to become what Fuggetta calls a "brand advocate" who will help you sell your product to other people. Capture the contact information for these customers, and you're well on your way to getting those easy-to-close referral sales.

2. Make it easy for your best customers to post reviews.

Once you've identified potential brand advocates, invite them to write reviews of your company or product on sites that post customer reviews (e.g., Yelp, Amazon, Best Buy) and provide them with links to the appropriate pages on those review sites.

According to Fuggetta, about 20% of customers who answered "9″ or "10″ will write reviews. Therefore, if you've got 10,000 customers and 2,000 answer "9″ or "10," you'll end up with 400 customer reviews. That's a lot of reviews.

Customer reviews accomplish two things. First, they make your product more attractive to new buyers, because buyers are more influenced by the opinions of their peers than your advertising or marketing. Second, when existing customers make a public commitment to your product, they are more likely to follow through on the next three steps.

3. Encourage your best customers to write testimonials.

Reviews talk about the product and how well it works. Testimonials are personal stories about how the product "changed the world" for the customer or the customer's firm. (Think "Kodak moment.") Testimonials are much more useful than reviews as sales tools because stories tend to stick in people's minds.

Fuggetta cites the example of a restaurant called Rubio's that reputedly sells great fish tacos. While Rubio's has plenty of Yelp reviews to that effect, what sticks in the mind is the woman who named her daughter Ruby because she went into labor while eating a fish taco at the restaurant.

When you get a testimonial, you post it on your website, of course. However, testimonials are even more powerful as sales tools when customers post them on their Facebook pages, especially if they're willing to include a link to your website.

4. Invite your best customers to answer questions from prospects.

Prospects often have questions before they buy. Though your sales team can probably answer those questions, those answers will be taken more seriously when they're provided by one of your existing customers. There are two ways to make this happen.

The cheapest method is to have a support forum in which customers can answer questions. The trick here is that it has to be easy for prospects to register and ask questions. Also, you'll need to keep an eye on the forum to make sure that dissatisfied customers don't start bad-mouthing.

A more elegant approach is to put a banner on your website reading: "Got a question? Ask a customer!" When a prospect clicks on the banner, your website sends requests to your network of brand advocates (who've given their permission for you to do this, of course) so that whoever is available can answer the question.

Fuggetta claims that using existing customers as advocates in this way has allowed one of Zuberance's clients to achieve astronomical sales conversion rates of 25%. In other words, one out of four prospects who ask questions of existing customers end up purchasing the product!

5. Give your best customers promotional offers to share.

Once you've developed your network, you can get them to share your special offers with friends and colleagues in the same way they share cute-kitty videos and interesting news articles, using whatever social networking platform they prefer.

Such offers can range from special discounts and coupons (for consumer products) to white papers and invitations to webinars (for B2B products). The trick here is to make the offers easy to share and have an easy way to give your advocates a heads-up that the offers are available.

Interestingly, such sharable offers are more effective when customers don't receive any compensation for sharing them. Turns out that giving existing customers special gifts or discounts makes their recommendations less likely to be heeded.

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