In the struggle to court new business, it is woefully easy to take your existing customers—the ones you already expect to see walk through your door—for granted. But even when your quality of service remains high, continued business from familiar faces is not guaranteed. According to a Texas A&M University study, the average business loses between 20 and 40% of its customers each year. But a lapse in patronage doesn’t necessarily mean that a customer is gone for good.

“The first area of concern is setting up a service and products to prevent customer loss,” says June Van Klaveren, owner of St. Louis-based marketing firm Compelling Communications, Inc. While committed service is always the best way to keep customers loyal, studies show that “lost” clients often cite “no special reason” for defecting. Life simply intervenes. But even if these customers had no real reason for leaving, you need to give yours a real reason to come back.

The results should be well worth the effort. Studies show that firms have a 20–40% probability of selling to a lost customer as opposed to a 5–20% chance of making a sale to a new prospect. Plus, since you’ve done business with these lost customers in the past, you’ll have a better idea of how to serve them once you get them back.

Start by prioritizing. “More attention needs to be paid to those who complain,” says Van Klaveren, “Go for the most influential customers—the ones who can do you the most harm— first.” Customers are far more likely to tell their friends about a bad experience than a good one, so any customer who complains could potentially damage your reputation with prospective clients. Van Klaveren also points out that only one out of every 26 dissatisfied customers speaks up. Listen to that one person carefully, acknowledge your mistake, and follow through on a promise to correct it.

It’s also your job to locate those who don’t complain. Van Klaveren recommends offering an incentive like a free gift or a discount to customers who complete user satisfaction surveys. This is a way to maintain a dialogue with clients and obtain their contact information. Once you have this, create a database of your most frequent customers and run a report every few months to see which haven’t made a purchase recently. When you notice that a once-valued customer hasn’t been in for close to a year, get in touch.

Many inactive customers may not even realize they’ve been inactive. Something as simple as a brief letter telling them you miss their business could be enough to bring them back. If it’s not, contact them again a few months later, advertising a special offer for valued customers. If even this fails, get in touch a final time asking for feedback. Find out why they left and what you could have done to keep them. But remember that no one takes well to hounding. “Set the number of contacts you make at four,” says Van Klaveren. “You should approach customers with an attitude of service, not desperation.”

You may find that the reasons for a customer’s defection were beyond your control; people’s needs change, or they move away, or they simply find that a competitor’s service suits them better. But knowing why a customer left, at the very least, is better than the alternative.