Friendships just aren’t as easy as knocking on a buddy’s door anymore. Not as easy as stepping outside and sipping iced tea with neighborhood moms while our children play.

No, as we age and as our lives get more and more frenetic, our friendships take more effort. We move. We marry. We have children. We change. Sometimes we just seem to run out of time.

“Friendship is the ball we drop with everything else we juggle,” says Marla Paul, 52, author of “The Friendship Crisis: Finding, Making, and Keeping Friends When You’re Not a Kid Anymore” (Rodale, $21.95).

Yet through years of experience, we know that our friends bring out our best and accept us at our worst.

If we’re knee-deep in life’s glop, they grab hold and yank us to our feet. They fulfill us in ways family, colleagues, careers and volunteer work cannot.

Friendship may sometimes seem just one more obligation. But as anyone who has ever spent time with a friend will tell you, it’s worth every extra effort.

For Laura Barbosa, of Plano, Texas, and eight friends, all in their early 20s, friendship means getting together each month. They might roller skate or eat dinner or arrange a slumber party – even though it would be easy to use new jobs, relationships and family situations as excuses not to.

When Paul moved to Chicago from Dallas a decade ago, she’d see groups of moms chatting in car pool lines at her daughter’s school or in the auditorium during a holiday performance. She longed for the friendships that seemed such a part of every woman’s life but her own.

Yet when she wrote a column for the Chicago Tribune about her dearth of friends, and was embarrassed to do so, “I got this huge response,” she says.

“Everybody said the same thing: ‘Thank God it’s not just me.’ “

The response was similarly overwhelming after she wrote an essay on the topic for Ladies’ Home Journal.

In her book, Paul offers suggestions about how to make new friends and how to nurture those new relationships, as well as those you may have let slide. “You have to be very proactive about it,” she says.

Kristy Hurst and her 30-something friends – some married, some single, some career women, some stay-at-home moms – make a point of meeting every Monday night. They started the tradition about 10 years ago, when they used to watch “Melrose Place” together.

Now, every Monday morning, an e-mail goes out telling where and when to meet. Anywhere between four and 15 women tend to show up.

“It has kept us all close for many years now,” says Hurst, 35, who lives in Irving, Texas.

Mary Blasingame Wimberly and Sue Thompson met when they were 13 and have been friends for 52 years.

They finish each other’s sentences, laugh at each other’s (good or bad) jokes. Die-hard antiquers, they planned a trip two years ago to Florida.

But at the last minute, Thompson, who lives in Denton, Texas, broke her ankle and said she couldn’t go.

Wimberly wouldn’t let her stay home. They rented a wheelchair and took off. After the trip, “the docs were amazed at her foot,” says Wimberly, who lives in Fort Worth, Texas. “The circulation was incredible and veins that had been broken down had developed.”

That’s friendship for you. It’s no coincidence we feel better when we’re with our friends; we really are better.

“Friendships protect us against depression,” Paul says. “They boost our immune system, enhance our memory. People with friends get fewer colds; they sleep more deeply … you just know how you feel after spending time with a friend: energized, happier, less burdened if you’ve shared a problem.”

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