She's been married for nearly 70 years. Today, her No. 1 rule for a lasting relationship is simple

Rosalyn Engelman was just 15 when she met her future husband, Irwin, she says. That was in 1953.

The two were set up by a mutual friend, and when Irwin, then 19, came to pick her up at her family's home in the Bronx, "I certainly had not dated anyone like the tall, dark, handsome man in a navy-blue suit with a briefcase who came to my door," she says.

"He looked like a movie star." Irwin took her out for a movie and milkshake, and by the end of the night, they were falling in love, she says. After three years of dating, the two got married in November 1956.

In their nearly 70 years together, they had two daughters, stood side by side through life-threatening illnesses, traveled the world, and built their careers. Irwin worked as a CFO at companies like Xerox, and Engelman was a painter and mixed media artist whose work has been displayed all over the world, she says.

They now live at the Apsley, an assisted living facility in Manhattan.

For couples seeking tips on how to stay happily together for decades, here's Engelman's advice.

For Engelman, it really comes down to one piece of advice: "Try to understand the other person," she says.

That can come into play in many ways.

If your spouse sometimes prioritizes their work, for example, put yourself in their shoes. "I never resented his time that he worked hard," she says, "and I don't think he resented the fact that I was covered in paint."

Try to get interested in their hobbies as well. "He liked opera more, I learned to like opera," she says. "I like classical music most, he learned to love classical music."

Finally, understanding each other can also mean forgiving each other during mishaps, large or small. Engelman remembers the first time she tried to cook the two of them dinner. She decided on Brussels sprouts, hot dogs, and corn. And the dinner did not come out as she planned. The Brussels sprouts tasted like rubber, she says.

Irwin was not angry. Instead, "we just started laughing and went out for pizza," she says. That's the kind of mutual understanding that can keep a relationship strong.

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