We really aren’t very good at forecasting the future, but one thing is predictable year after year—most people who set a New Year’s resolution will fail to keep it.

Gyms bank on it. Gym owners know that most of the people who sign up for a membership in January will not show up regularly within weeks. One survey of 5,300 gym members found that 63% of memberships go completely unused.

It’s too bad because most resolutions—if kept—lead to positive changes in our lives. Whether you’re making a commitment to eat healthier, exercise more, or make more sales calls for your business, goals propel you forward and are worth pursuing.

In their new book, The Power of Bad, bestselling authors John Tierney and Roy F. Baumeister offer a rule of thumb to help you reach any goal you set your mind to—including New Year’s resolutions.

 
 

The authors call it the ‘Rule of Four.’ Simply put, it takes four good things to overcome one bad thing. “The typical person has three good days for every bad day. To do better than average, then, you’d want to have at least four good days for every bad one,” write the authors.

According to the authors, don’t set up unrealistic scenarios when you start a self-improvement plan. You’re likely to fall short of your expectations from time to time—and that’s okay. It’s more important to get back on the horse than to give up completely.

Numerous studies have found that people give up on their goals very quickly after the first time they slip up. Nutrion researchers call it the ‘what-the-hell’ effect or ‘counter-regulatory behavior.’ It describes a vicious cascade of counter-productive behavior the moment a person breaks their commitment.

For example, if a dieter tries too hard to avoid ever having a piece of cake—and breaks down at a party to indulge in a slice of delicious double-chocolate cake with buttercream frosting—the dieter despairs and eats another piece, and another, and another.

The key is to break the cycle with the following mental hack.

“Instead of demanding perfection and despairing when you fail, you could aim to stick to your regime at least four days out of five,” write Tierney and Baumeister. While the authors acknowledge that some goals—like quitting smoking—might require a cold-turkey approach with no wiggle room—most goalsetters benefit by following a “virtue-to-vice ratio of 4 to 1.”

The Rule of Four is a powerful goal-setting concept because it recognizes a reality of human nature: most of us overestimate the amount of willpower it takes to stick to a goal. We also fail to consider the demoralizing effect of a small setback. This comes back to the central theme in The Power of Bad. Many studies show that one negative event or setback has much more impact than a positive one—at work, in our personal lives, and in the goals we set for ourselves.

Break the cycle by making sure your good days outweigh the bad.

Read on Forbes.com