En español | We all know couples who've been married for 30, 40, 50 years or more — and who seem as genuinely happy as they were when they were newlyweds.

Quiz: How healthy is your marriage?

Displays of affection are important to a successful relationship. — Donna Day/Big Cheese Photo/Corbis

I get such pleasure out of seeing these lovebirds, and I observe closely to see what traits and behaviors might be the key to their relationship success. Over the years, I've concluded that these are the five habits of happily partnered couples:

1. They keep up with the changes. Lillian Hellman once said, "People change and forget to tell each other." When it comes to marriage, that can be risky. The most successful couples really take note of each other's changes. They do not assume their partner is the same person he or she was 20 years ago, even if there are many similarities. What's more, they take the time to learn their partner's goals, dreams and future plans. By keeping in touch with who their partner is at this moment — and looking ahead to who he might become — they secure a truly intimate relationship.

 2. They know how to fight fairly. It's not that happy couples never argue. Most couples have disagreements. But in a mature relationship, power isn't defined by winning an argument or getting one's way. True power comes from knowing how to discuss differences fully and honestly. If you demean your partner when you disagree, and if, at the end of an argument, you do not feel stronger and more intimate than you did before you started — you are not building a stronger, more loving relationship. Successful couples know how to argue with class and dignity. They may disagree, but in the end, they end up understanding — and respecting — their differences.

3. They find new ways to play. All the research on marital satisfaction shows that couples bond more closely when they do new, innovative activities — instead of getting stuck in the same rut they've been in for the past 25 years. Whether it is learning how to sculpt together, opening an inn, signing up for the Peace Corps, or simply helping each other create a healthier lifestyle, any kind of new, enjoyable pursuit can make a couple that feel younger and more in sync — and can invigorate their love.

Next: Stay physically connected. »