With average life expectancy now approaching 80, Americans can look forward to spending almost two decades enjoying retirement. That free time can seem heavenly at first—until the days stretch on. "[After] the honeymoon stage comes the disenchantment stage," says Dr. Sara Yogev, psychologist and author of "A Couple's Guide to Happy Retirement." "People feel like everything is purposeless. They can get depressed, and we would like to avoid that stage."

Discovering your purpose—your driving force—is a proven way of escaping that emptiness. "From what we know from research, those that have a sense of purpose are happier," says Dr. Yogev. "Their adjustment to retirement is better and their marriages are happier." What’s more, studies show that retirees with a defined, actionable purpose have improved cognitive function and a reduced risk of stroke.

Of course, uncovering your fundamental motivation is no small task. We asked Dr. Yogev for tips on how to get the ball rolling.

1. Write your values and interests down
Nothing helps crystallize your thoughts like seeing them on paper. Take time to contemplate your strongest-held interests and central beliefs. Then, rate them. No, really. "Put them on a scale and rate them 1 to 10," suggests Dr. Yogev. This exercise will help you prioritize what’s most important to you, which can guide you toward your essential purpose.

#2. Ask yourself how you want to be remembered
Contemplating your legacy—the accomplishments that live on after you pass—is key in resolving your reason for being. "Imagine your grandchild is talking to his own grandchildren about you. What is said? Would he say you were generous and kind? Would he say you were reliable?" asks Dr. Yogev. These questions can help guide you towards finding a purpose for which you will be proud to be remembered, and is compatible with who you are and how you are seen by others.

To help this exercise along, remember your own mentors and idols, and consider modeling your behavior after theirs. "Think about people you admire and respect. Who are they? What is it that they do that might be realistic for you to do, in some way?"