When people fall in love and decide to marry, the expectation is nearly always that love and marriage and the happiness they bring will last; as the vows say, till death do us part. Only the most cynical among us would think, walking down the aisle, that if things don’t work out, “We can always split.”

But the divorce rate in the United States is half the marriage rate, and that does not bode well for this cherished institution.

While some divorces are clearly justified by physical or emotional abuse, intolerable infidelity, addictive behavior or irreconcilable incompatibility, experts say many severed marriages seem to have just withered and died from a lack of effort to keep the embers of love alive.

I say “embers” because the flame of love — the feelings that prompt people to forget all their troubles and fly down the street with wings on their feet — does not last very long, and cannot if lovers are ever to get anything done. The passion ignited by a new love inevitably cools and must mature into the caring, compassion and companionship that can sustain a long-lasting relationship.

Studies by Richard E. Lucas and colleagues at Michigan State University have shown that the happiness boost that occurs with marriage lasts only about two years, after which people revert to their former levels of happiness — or unhappiness.

Infatuation and passion have even shorter life spans, and must evolve into “companionate love, composed more of deep affection, connection and liking,” according to Sonja Lyubomirsky, a professor of psychology at the University of California, Riverside.

In her new book, “The Myths of Happiness,” Dr. Lyubomirsky describes a slew of research-tested actions and words that can do wonders to keep love alive.

She points out that the natural human tendency to become “habituated” to positive circumstances — to get so used to things that make us feel good that they no longer do — can be the death knell of marital happiness. Psychologists call it “hedonic adaptation”: things that thrill us tend to be short-lived.

So Dr. Lyubomirsky’s first suggestion is to adopt measures to avert, or at least slow down, the habituation that can lead to boredom and marital dissatisfaction. While her methods may seem obvious, many married couples forget to put them into practice.

Building Companionship

Steps to slow, prevent or counteract hedonic adaptation and rescue a so-so marriage should be taken long before the union is in trouble, Dr. Lyubomirsky urges. Her recommended strategies include making time to be together and talk, truly listening to each other, and expressing admiration and affection.

Dr. Lyubomirsky emphasizes “the importance of appreciation”: count your blessings and resist taking a spouse for granted. Routinely remind yourself and your partner of what you appreciate about the person and the marriage.

Also important is variety, which is innately stimulating and rewarding and “critical if we want to stave off adaptation,” the psychologist writes. Mix things up, be spontaneous, change how you do things with your partner to keep your relationship “fresh, meaningful and positive.”

Novelty is a powerful aphrodisiac that can also enhance the pleasures of marital sex. But Dr. Lyubomirsky admits that “science has uncovered precious little about how to sustain passionate love.” She likens its decline to growing up or growing old, “simply part of being human.”

Variety goes hand in hand with another tip: surprise. With time, partners tend to get to know each other all too well, and they can fall into routines that become stultifying. Shake it up. Try new activities, new places, new friends. Learn new skills together.

Although I’ve been a “water bug” my whole life, my husband could swim only as far as he could hold his breath. We were able to enjoy the water together when we both learned to kayak.

“A pat on the back, a squeeze of the hand, a hug, an arm around the shoulder — the science of touch suggests that it can save a so-so marriage,” Dr. Lyubomirsky writes. “Introducing more (nonsexual) touching and affection on a daily basis will go a long way in rekindling the warmth and tenderness.”

She suggests “increasing the amount of physical contact in your relationship by a set amount each week” within the comfort level of the spouses’ personalities, backgrounds and openness to nonsexual touch.

Positive Energy

A long-married friend recently told me that her husband said he missed being touched and hugged. And she wondered what the two of them would talk about when they became empty-nesters. Now is the time, dear friend, to work on a more mutually rewarding relationship if you want your marriage to last.

Support your partner’s values, goals and dreams, and greet his or her good news with interest and delight. My husband’s passion lay in writing for the musical theater. When his day job moved to a different city, I suggested that rather than looking for a new one, he pursue his dream. It never became monetarily rewarding, but his vocation fulfilled him and thrilled me. He left a legacy of marvelous lyrics for more than a dozen shows.

Even a marriage that has been marred by negative, angry or hurtful remarks can often be rescued by filling the home with words and actions that elicit positive emotions, psychology research has shown.

According to studies by Barbara L. Fredrickson, a social psychologist and professor at the University of North Carolina at Chapel Hill, a flourishing relationship needs three times as many positive emotions as negative ones. In her forthcoming book, “Love 2.0,” Dr. Fredrickson says that cultivating positive energy everyday “motivates us to reach out for a hug more often or share and inspiring or silly idea or image.”

Dr. Lyubomirsky reports that happily married couples average five positive verbal and emotional expressions toward one another for every negative expression, but “very unhappy couples display ratios of less than one to one.”

To help get your relationship on a happier track, the psychologist suggests keeping a diary of positive and negative events that occur between you and your partner, and striving to increase the ratio of positive to negative.

She suggests asking yourself each morning, “What can I do for five minutes today to make my partner’s life better?” The simplest acts, like sharing an amusing event, smiling, or being playful, can enhance marital happiness.