The New Year is a reminder that we're our own worst enemies.

In fact, a study of one million people around the world revealed that most people believe self-control is their biggest weakness.

But willpower is like a muscle: it can be strengthened — and fatigued — with use.

"You only have a finite amount as you go through the day, so you should be careful to conserve it and try to save it for the emergencies," says New York Times author John Tierney, who co-authored a book with social psychologist Roy Baumeister, "Willpower: The Greatest Human Strength," which looks at how willpower and decision-making are interconnected.

We pulled out some of the best takeaways, which Tierney covered in two excellent pieces for the Times ("Do You Suffer From Decision Fatigue?" and "Why You Need To Sleep On It").