It’s now an old story, and by no means confined to retail: Many apps wind up downloaded, used once and forgotten—or worse, deleted. When it comes to mobile commerce apps, retailers need to figure out how to keep shoppers engaged with their apps so shoppers keep coming back for more.

The newly-published 2015 Internet Retailer Mobile 500 features a great many new m-commerce metrics this year, including app engagement, which is the frequency with which shoppers who have downloaded an app use it. The 2015 Mobile 500 ranks the leading 500 retailers worldwide in mobile commerce by annual mobile sales. 42% of all mobile sales generated by the leading 500 merchants in m-commerce in 2014 will come from mobile apps, the Mobile 500 finds.

Mobile app analytics vendor Mobidia Inc., an official Internet Retailer data partner for the 2015 Mobile 500, exclusively measured app engagement levels of the 262 merchants in the Mobile 500 that offer apps. Mobidia used its panels of mobile app users, 250,000 consumers in the U.S. and 2 million worldwide. For the purposes of the Mobile 500, engagement is expressed by the percentage of monthly active users (users who shop an app at least once a month) who shop an app at least once a week.

Mobile 500 retail apps with the highest engagement levels include Neiman Marcus (64.3%), Kohl’s (63.6%), Victoria’s Secret (61.7%), H&M (56.6%) and Groupon (47.8%). Kohl’s, for example, does a great job of including features and offers in its app that keep customers coming back for more, says Chris Hill, vice president of marketing at Mobidia.

“Kohl’s highlights discounts, coupons and deals of the day; retailers with apps need to be doing these kinds of things to give people a reason to come back to the app,” Hill says. “A recognized chain name can help boost engagement, but only if the app is designed well and gives shoppers a reason to return. An app is an opportunity for a brand to leverage its most loyal customers.”

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Belk Inc., a mass merchant retail chain that’s No. 171 in the 2015 Internet Retailer Mobile 500, has a substantial level of app engagement. 32.4% of the Belk shopping app’s monthly active users use the app at least once a week, according to the Mobile 500. Belk offers apps for the iPhone, iPad, Android smartphones and Android tablets.

“Mobile apps are becoming much more important as digital commerce evolves,” says Ivy Chin, senior vice president of e-commerce and omnichannel digital at Belk. “Our shopper often uses her mobile device to complement her in-store experience, so we developed the Belk app to support that, with easy bar code scanning, coupon integration with Apple’s Passbook wallet, a Belk store finder, integration of e-mail, and social sharing.”

Belk is examining Apple’s iBeacon technology for potential future integration into its app. Beacons are small pieces of hardware that retailers place throughout a store that communicate with apps on mobile devices via Bluetooth Low Energy wireless networking technology, so no Internet link is required. A beacon can, for example, sense that a shopper with the Belk app on her smartphone just entered a particular department and transmit to her smartphone via app push notification a coupon for 20% off any purchase in that department. An app does not need to be open to receive the message, which triggers an alert tone. An app user would, however, need to have approved the use of push notifications, as well as have her Bluetooth on, which is quite common with the advent of the low-energy version of Bluetooth.

“It’s true that retailers have yet to fully make use of location in mobile commerce, but you have to do location right,” Chin says. “Location awareness has to be meaningful for customers and give them the right information and the right messages. This involves understanding how your customers engage with your brand whether they are in-store, online or on a mobile device—a 360-degree view of your customers.”

Follow Bill Siwicki, editor of the 2015 Internet Retailer Mobile 500 and managing editor,mobile commerce, at Internet Retailer, at @IRmcommerce.