Since pay-now, work out-later isn’t working as a strategy, economists have been searching for other solutions to get people into the gym more often.

“We describe a field experiment measuring the impact of bundling instantly gratifying but guilt-inducing ‘want’ experiences (enjoying page-turner audiobooks) with valuable ‘should’ behaviors providing delayed rewards (exercising),” wrote the economists Katherine Milkman, Julia Minson and Kevin Volpp of Wharton in a 2013 research paper. They gave Penn undergraduates free iPods loaded with audiobooks of their choice, but told them they could listen to them only at the gym.

This worked for a while: The undergrads given the iPods went to the gym about 50 percent more often than others who were just given a Barnes & Noble gift card to spend as they pleased. (A third group, given a book iPod to take home but encouraged to listen only at the gym, placed in between the other two.) But that lasted for about seven weeks. Thanksgiving break came, and the students returned to school having fallen out of love with their audiobooks; they no longer worked out more than their peers in the control group.

“It was a bummer from the perspective that we’d like to design interventions that work forever,” said Ms. Milkman. But she also noted that there are lots of opportunities to try again. Gym visits don’t just pick up at New Year’s but at the start of each week, each semester, after holidays, and even after birthdays (though not 21st birthdays). The key is getting one of the fresh starts to stick.

Heather Royer, an economist at the University of California, Santa Barbara, thinks she has hit on a way. In partnership with a Fortune 500 company, she and her research partners ran an experiment that combined two interventions: One to get people to start going to the gym, and another to keep them there. For four weeks, the company paid its employees to work out, $10 per visit up to three times a week. After those four weeks, there were no more payments, but some workers were offered a “commitment contract”: They could set aside their own money that would be released to them only if they worked out over the next two months; otherwise, it would be given away to charity.