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By Rheana Murray

In a longtime relationship, it's easy to let comfort and routine get in the way of passion — but don't.

Because today is Valentine's Day, and Dr. Ruth Westheimer, the 90-year-old renowned sex therapist — "90 and a half," as she'd probably tell you — wants to remind people that being a part of a couple doesn't mean romance has to die.

The biggest mistake she sees couples make?

"Putting sex on the back burner," Dr. Ruth told TODAY. "Make sure you really look forward to having sex with your partner. Stop saying it's not as interesting as it was 20 years ago, but say how fortunate you are that you have a partner in life."

And another thing: Check that iPhone at the door.

Dr. Ruth poses backstage during New York Fashion Week this month. At 90 years old, the therapist is still a go-to expert for sex and relationships. Astrid Stawiarz / Getty Images

"Leave your cell phones outside the bedroom, because you have to talk to your partner," she said. "If you pick up one thing that is the most important, it's communication. You have to talk to each other."

Of course, adding children to the equation makes romance all the more difficult. In fact, in TODAY's Valentine's Day online survey, most parents said they'd take sleep over sex. But still, Dr. Ruth says it's important to find the time.

"To be realistic, older people don't have the same stamina as younger people," she said. "And parents particularly have a difficult time finding time, so I tell them loud and clear: Go check into a motel. You don't have to stay the whole night. Check in for a few hours. Take a bubble bath, have some champagne, a little dinner, some good sex. Then go home and pay the babysitter."

Dr. Ruth's incredible story and rise to sex therapy stardom is the plot of an upcoming Hulu documentary about her life. (Here's a trailer, but warning: Some of the content is NSFW.)

She spoke to TODAY on behalf of her campaign with Mr. Peanut, who recently helped her bring back her hotline, just in time for Valentine's Day. While many people, especially those who have been in committed relationships for some time, are quick to brush Feb. 14 off as a "Hallmark holiday," Dr. Ruth suggests people view the day as an opportunity to reconnect.

"It is commercial, but so what?" she said. "It's a fun commercial holiday. Go and buy a card. Or make a card. Go buy some flowers. You can never bring too many flowers into a home. Do something you haven't done in a while."