A few days into the new year and most of us are already beginning to waver in our earnest resolve to skip dessert, hit the gym every morning, and finish War and Peace. It's a well-known phenomenon that recurs every year and that economists have formalized into a theory called hyperbolic discounting.

"We are more optimistic about our future and our future selves than we are in the present," says David Rose, an entrepreneur and founder of Vitality, a medical-monitoring startup. In other words, it's easy to make plans about how healthy, responsible, and efficient we will be next week but hard to execute them once next week becomes today.

As behavioral economists—who use social, cognitive, and emotional factors to understand how people make choices—refine their understanding of what helps us stick to commitments, they are using this information to design new tools. Not surprisingly, money turns out to be a good motivator.

Ian Ayres, a behavioral economist at Yale, developed a website called StickK.com, on which users set a specific goal and then pledge a sum of money to forfeit should they fail to achieve it. Unlike other sites that track weight loss and fitness goals and offer support via social networking, StickK leverages another discovery from behavioral economics: our extreme dislike of losing money.

"The specter of losing money is twice as motivating as the possibility of gaining the same amount of money," says Rose. Users can even designate an "anti-charity" as the beneficiary of their money should they fail to meet their goals. For example, someone favoring restrictions on gun ownership might choose the National Rifle Association.

The site, which is entering its third year, has more than 50,000 users who have put about $5 million at risk. "We have a 70 to 80 percent success rate in helping people stick to serious commitments to exercise or lose weight," says Ayres, who recently published a book, Carrots and Sticks, which lays out the behavioral economics behind crafting incentives.

Interviews with StickK users and analysis of data from the website show that an even more important motivator than money is accountability. Users can designate referees to monitor their progress and make sure they are reporting it accurately. They can also choose to make their quest public, adding a worldwide sense of accountability.