By now everyone knows that Millennials’ buying habits vary from those of other age groups. But understanding they differ has become more crucial now that Millennials dominate as the largest consumer demographic—bigger than the baby boomers—and account for 21 percent of all consumer discretionary spending.

What kinds of messages work with this younger audience?

It probably can’t be repeated often enough, but a mobile-friendly approach is essential. “It could be as simple as having a mobile-friendly website,” says Andrew Robbins, president of Paytronix, which creates loyalty programs, gift card strategies and email marketing for a number of restaurants and chains.

“Millennials have a mobile device with them at all times—most of us do, but Millennials grew up with them and are tied much more closely to them,” he adds.

Targeting younger diners requires communicating with them in a style that resonates with them. A mobile app that provides relevant information and pushes messages to the phone’s lock screen, for instance, is more effective than emails. Why? Many Millennials simply delete or ignore emails.

Being relevant matters to Millennials, who are loyal to brands that understand them. And Robbins says it isn’t just the message the brand sends, it’s how that message is conveyed. “Different groups like to be spoken to in a different tone of voice; Millennials may want a more casual, even cheeky, tone,” he says.

On the other hand, leaning too heavily on a mobile approach is a mistake. Recent Paytronix research on Millennials shows that many of them actually prefer to use a loyalty card: “It works, it’s simple and you can’t track me,” they reason. “If you only do a mobile loyalty program, you may actually irritate a third of Millennials,” Robbins suggests.

Social media is another powerful way to reach Millennials. “We have seen some success with viral marketing, especially refer-a-friend-type programs for fans,” Robbins says. Promoting programs like that on Facebook lends them legitimacy. “If people see the idea posted a lot, they are more likely to believe it than if it comes from a single source,” he notes.

What about other, more traditional techniques for getting customers through the door more often? “The real key is to understand the guest and be able to personalize offers,” Robbins says. His company’s research has shown that Millennials don’t want to go to the trouble of printing and redeeming a coupon, for instance. What works for them instead?

“By far the reward they prefer is a free product,” Robbins says. “They would rather spend $100 and get a free pizza.” The next-most-valued promotions were buy one/get one, member-only special pricing and gift-with-purchase deals, such as a gift card for a discount on a future meal.

What’s the underlying message Millennials are sending? “Don’t make me work for it.”