Linda Lajterman suffered one of the worst experiences of her life while on a cruise with her husband and two other couples. Halfway through the trip, one of her friends stopped talking to her—for good.

Ms. Lajterman says she has no idea what prompted the woman, who was one of her best friends, to cut her off. They helped take care of each other's kids, celebrated family events together and shared confidences. After the cruise, which took place a few years ago, she called her friend and asked for an explanation, but received none. She says she was devastated.

"I would have welcomed the opportunity to apologize or discuss it if I did anything wrong," says Ms. Lajterman, a 52-year-old nurse from Ramsey, N.J. "Instead, it took me three self-help books and two years to make peace with the fact that someone I thought was a good friend ended our friendship."

There are 50 ways to leave your lover, according to Paul Simon. But how many ways are there to leave a friend?

I know, it's a terrible question. But think about it: Some of the worst breakups in our lives are not with romantic partners. They are with friends—the people with whom we often share our deepest thoughts. Friends provide guidance, encouragement, laughter and a refuge. Losing a good friend can be one of the saddest experiences in life.

And yet, many friendships just don't last. Some simply fizzle out, victims of routine life events such as moves, job changes, divorce or a divergence of interests.

Others end badly. Rob Wilson, 53, a writer in Atlanta, saw a 12-year friendship abruptly end after he mentioned he was voting for George W. Bush in the 2004 presidential election. Arthur Newton, 46, a hotel manager from Austin, Texas, had a female friend tell him she couldn't hang out with him anymore because her husband was jealous.

Michael Hassard watched a good friend run away from him—literally. He had heard his pal had begun dating his ex-girlfriend, so Mr. Hassard, 39, a NASA engineer from Muscle Shoals, Ala., approached him in church one day to ask about it. But before he could speak, his friend turned and fled down a hallway, out the door and into his car. He and his former buddy never spoke again.

Friendships are such a nuanced and intriguing relationship that we even follow celebrity friend breakups, as we do their romances. Why else would we care about Mariana Pasternak but for her tell-all book about her former friendship with Martha Stewart, which ended after Ms. Pasternak testified at Ms. Stewart's 2004 trial.

"It's a myth that friendships last forever," says Irene S. Levine, a psychologist, professor of psychiatry at New York University's medical school and author of "Best Friends Forever: Surviving a Breakup with Your Best Friend." We are tied to our family by blood and our spouses by law, so we are often more attentive to those relationships. "Friendships are relationships of choice, so we tend to overlook them," she says.

As a result, many friendships die from neglect, Dr. Levine says. And this in itself poses a very sticky problem in friendship breakups: How do you know if you're being neglected—or dumped? What if your friend is always too busy to get together but always seems to have a good excuse? What if she never calls you, but seems happy enough to hear from you when you call?

And there's the rub. There are no rules or even societal norms for friendship breakups. Friends who want to split don't go to counseling or get a mediator or a lawyer, as divorcing couples do. And there typically aren't a bunch of nosy relatives willing to intervene and relay messages, as there are when a split is within a family.

Also, dissolving a friendship is harder than ever these days, with so many digital ties holding us together, from social-networking Web sites like Facebook to stored numbers in cellphones.

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Dave Nadkarni can tell you all about it. When he decided to end a relationship a few years ago with a close female friend he felt was spreading rumors about him, he stopped returning her calls, defriended her on Facebook, blocked her on his instant-message list, stopped following her on Twitter and changed her name in his cellphone to "Do Not Pick Up." "It was cathartic," he says.

But it didn't work. His friend got the hint and stopped calling him, and he has successfully avoided seeing her in real life. But he still runs into her constantly online, every time a mutual friend retweets her Twitter posts or she leaves a comment on a mutual Facebook friend's status update.

"It sucks," says Mr. Nadkarni, 29, a sales rep for a security company in Las Vegas. "It's like the dog that's stuck on your leg that you can't shake off."

So how do you finish off a friendship? Are some ways better than others?

Psychologists recommend ending a friendship

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