Last year, Candice Galek, the founder of online swimwear company Bikini Luxe, started using an app to send out automated emails thanking customers for their purchases. To her surprise, many replied.

"Our customers were really responding to it, writing back and thanking us for the message," Galek says. So she decided to push things even further and tried sending handwritten notes to a small group of buyers. The results astonished her: More than half of these customers returned to the site to buy again within the year.

"Pretty stunning," she says. "It absolutely kills our email" response rates.

The Power of Returns The math behind what makes your most reliable customers so valuable: 8 percent The average part of a U.S. business's customer base made up of longstanding buyers. 41 percent Portion of total U.S. online revenue that returning customers account for each year. 5 percent Increase in customer-retention rate at financial firms that resulted in a profit growth of at least 25 percent. 7 bucks How much it costs for a company to market to an existing customer. Marketing to a new customer costs $34. 20 percent The amount by which the spending of a repeat customer exceeds that of a first-time buyer. Sources: Adobe, Harvard Business Review , SumAll, Boston Consulting Group

The road to a successful business becomes much easier if you have lots of repeat customers riding with you. The consultancy Bain & Company finds that repeat customers spend more: For apparel companies, a shopper's fifth purchase is, on average, 40 percent larger than the first.

But keeping users is harder than it looks. Just getting a customer to come back to an app is never guaranteed--how many times have you down­loaded one and never used it?--and most companies are not very good at it, says Kamo Asatryan, founder and CEO of mobile app consultancy and developer Primer. He estimates that around 80 percent of people who download an app try it only once.

To combat this, consider taking some cues from subscription-based personal-assistant service Fancy Hands. It starts by sharing ideas with new users on what tasks they should assign to their assistants. It sends a look-ahead email to sub­scribers--who can customize the frequency--that scans a calendar to suggest potential tasks to take on that week, like the birthday present you need to buy, say, or the doctor's appointment you need to make. That useful email, says Fancy Hands CEO Joshua Boltuch, is opened by 39 percent of subscribers who receive it. That trounces average industry email open rates.

Be sure to recognize the patterns in your customers' behavior. Food-delivery service Grubhub's extensive data on its users' order histories means it knows which ones are what it terms "weather-sensitive." It then monitors local weather patterns across all major markets and sends dedicated email prompts to such customers--and partially credits this move for the spike in orders it sees on rainy days.

It also helps to give your customers the chance to say yes on their own schedule. To Jason Sylva, executive director of digital media at The New York Times, winning readers and subscribers and keeping them on board is a major concern. Recently the Times offered some online readers a calendar reminder to subscribe later. The result? A 97 percent increase in people who subscribed later compared with those who were not offered the calendar reminder.

You can find new ways to work with what you know about your users to better target them. Spotify's Discover Weekly play­lists, which select new tracks for customers on the basis of their listening histories, have led more than 71 percent of users to "save" at least one track from those playlists.

And, of course, nothing beats figuring out what works and sticking to it. Galek is planning to send many more handwritten notes; now, she does so for only 5 percent of her customers. "The reality is," she says, "we should be doing it for as many people as possible."

Show them what they're missing.

Don't miss a chance to tell customers why they need your service. News-reading app Flipboard sends users a regular email about news stories on its platform, and recently launched a politics-related email. In the past year, its average number of users has more than doubled, from 42 million to 85 million.

Turn it into a game.

After they order, Grubhub users can play Yummy Rummy for discounts and freebies, and players who order five or more times a month are eligible to win a year of free food. "It's making people come back to order more," says CMO Barbara Martin Coppola.

Build a community.

DietBet, a service that allows users to bet on their ability to lose weight, is "almost like a big AA meeting," says founder and CEO Jamie Rosen. Users commiserate on their diets and struggles, and celebrate their successes. And they come back: DietBet sees a 35 to 40 percent repeat rate on its visitors.

Be thoughtful.