Develop self-control. Specifically, develop the ability to delay gratification, forgoing immediate indulgences in favor of long-term benefits. In practical terms, that often means getting things done when they need to be done, even when you’d rather be doing something else. It means resisting impulsive flings that will undermine your future happiness. In other words, learn to say “no” to you inner toddler.
This has been one of the most compelling findings of modern psychology: no other single factor is more determinant of success in school, career and relationships. Life just tends to unfold more smoothly for those who know how to just do what needs doing, and resist doing what they know isn’t the best choice.
This is a skill that can be developed at any age, but it’s easier when exercised during the teen years, when neural connections are forming between the prefrontal cortex and the rest of the brain.
Here’s a layman’s summary of some of the psychological research.