WE ARE ON the verge of losing an irreplaceable natural resource. The inexorable process of human aging is depriving us of one of the most extraordinary groups of human beings that has ever lived: America's older generation. The last veteran of World War I has died; those of World War II are now in their 80s. The youngest children of the Great Depression have reached their late 70s. When this generation has passed, where will we go to recover the lessons they learned about life? For five years, I have collected the advice and wisdom of over a thousand of our elders — "experts," as I call them — about surviving and thriving in a difficult world. Here is what eight of them had to say.

Marry someone like you

APRIL STERN *, 71, and her husband, Steve, were married for 47 years, until Steve's death. April is a highly respected community leader who directed several local organizations, and Steve was a well-known local psychotherapist. They were deeply in love throughout their long relationship. "I think we modeled a good marriage, our children even talked about that as being important to them," says April.

"It sounds simple, but you have to like each other. Be friends, try to get past the initial heaving and panting, and make sure there's a real friendship underneath that. I don't think you have to have identical interests, but you've got to have shared values. That is quite important. That was critical. Yeah, I think values are probably the most important thing.

"And we both loved certain kinds of things. We both loved movies, good movies, and part of our courtship involved staying up all night and figuring out what an Ingmar Bergman film really meant. We both loved to read, and we loved to talk about what we'd read.

"A similar sense of humor — that was a very important part of our life together. In fact, just two weeks before he died, we were talking one night, and he said something and I just dissolved in laughter, and he looked at me so self-satisfied and said, 'I can still make you laugh after all these years!' And he could."

Honor your vows

EUGENE EARNHART, 80, is a trim, handsome man with close-cropped gray hair. Eugene's speech is halting, due to the effects of a stroke, and it took him a while to tell me his most important life lesson. He did so with such emotion that he was forced to stop at times to collect himself. But in the end, he was relieved at the knowledge that others might profit from his experience. Eugene's career involved frequent relocation and extensive business travel, which was bound up with a life regret he cannot get over. He told me:

"Listen, this is very important and it's…it's that people should respect fidelity. I'm the worst one in the world to appreciate that because I was not a faithful husband, and I regret it. I think it was the fact that I roamed around the country in my work. And I really want to make this point about how fidelity is important to marriage. That's what I'd do over if you could. Oh, definitely! I was an idiot.

"And she was a wonderful wife. I could never make it up to her. Even to the end, I was unfaithful. Fidelity wasn't there. It's hard for me to say this, and sometimes I get really depressed about it. But I tell people, 'Don't ramble around the country, doing what I did.' Faithfulness is one of the most important things that people should cling to."

Share your kids' lives

CLAYTON GREENOUGH, 79, has very close relationships with his son and daughter, both of whom settled near him as adults. When asked for his lessons about child rearing, he reflected on the importance of going along with children's interests and making them shared activities.

"Maybe it's an old-fashioned way of speaking, but I feel that it's pretty important to stick with them. When my son was a sophomore in high school, I started putting up a shed in our backyard. And he was taking a shop class in high school at the time. I had him help me there, and before I knew it I'd come home from work and he'd be sitting with a toolbox, waiting to go ahead with some work. And this led him down a road where he actually saw the need for measuring and things like that and started to recognize that there is some value to arithmetic and mathematics. He eventually wound up being a mechanical designer. Now, if I hadn't been available to him at that time, I'm not sure what course he might have taken. So many of the things that he's doing now were initiated because we spent time together — the fact that there was somebody who was there and interested in what he was interested in."

Learn from bad experiences

SAM WINSTON, 81, trained as an engineer but also worked in marketing and as a general manager. He attributes his considerable career success specifically to his ability to learn from jobs he didn't like. The key, he says, is to see them as learning experiences and to take advantage of any opportunity to gather knowledge about an industry or occupation.

"One important thing for young people is to be observant. No matter what the task is, whether you like it or not, it's very important to learn everything you can about what's happening around you. You never know when that may be of great value later. I've had many different experiences throughout my life where I really didn't like what I had to do and I would feel what I was doing was inconsequential. But the lessons I learned doing those things played an important part in my life. For example, I had to work my way through college, in many jobs you may consider meaningless. Later on they were very valuable for me as an employer, to help me understand my people. I would tell younger people that no matter what the experience is — learn. See what's happening."

Choose happiness

MARIANNE RUMSEY, 76, has suffered from a discouraging number of health problems in recent years. She sees the sense of fragility that belongs to these experiences as directly contributing to her ability to savor life.

"With physical problems you have gone through, the day-to-day becomes more important because you don't know what tomorrow is. That became much clearer to me after I'd had my two or three heart episodes. Enjoy each day, if you feel fine today. There are no guarantees for the future. It sounds trite, but it's so true, really is, and we all forget about it, but it's important to remember. Lighten up, let's look at things and be happy instead of being glum and negative. Walk cheerfully on the face of the earth."

Stay connected

CHRIS SCHULTZ, 83, was raised a Lutheran in the upper Midwest, and his description of his small-town (population: 200) upbringing sounds a lot like Garrison Keillor's Lake Wobegon. He emphasizes the community he finds at church rather than his spiritual experience.

"I'm very active in a local church. We try to go every Sunday. But I'm not a particularly strong believer in the stuff that's associated with what churches say they're about, the afterlife kinds of questions, that sort of thing. There is one thing, though, that is important to me and it's the reason I participate. That is the sense of community that can come from people who also are concerned about the world. For people who don't take part in that church community, I think their lives are less rich. I grant that you can get it from other sources, but the church has some unique characteristics about it."

Accept life as it is

IN CECILE LAMKIN'S living room, a wall of windows looks out through the still-bare trees to a calm lake below. This house has been Cecile's home for over 50 years, only she recently gave up daily swims, she said, "Because I can't get down the stairs anymore." Widowed several years earlier after 68 years of marriage, Cecile, 92, explained that later life has brought her a sense of wholeness, acceptance, and the ability to enjoy small pleasures.

"I am much clearer now. I say that as an older person, not just as an adult, but as an older person, things are much clearer. I was just telling my daughter, I think I'm happier now than I've ever been in my life. And I've been thinking about why it is that I'm happier now. I came up with a lot of stuff. First of all, things that were important to me are no longer important, or not as important. The second thing is, I don't feel responsible in the same way that I used to feel. I've been a pretty responsible person, but I don't feel that responsibility anymore. My children are in charge of their lives, and whatever they do with them, they will do with them.

"And I live in a place, my house, that I love. In the summer here it is wonderful, and I live outdoors at that time. My family comes, friends come, and I use it like a vacation. I've also given up feeling that I have to entertain people. If there's someone coming up, they will bring such and such. It's very liberating for me. And I just feel a contentedness that I've never felt before. I've heard other people my age say the same thing."

Don't worry about dying

ROSEMARY BREWSTER, 90, is a regular churchgoer and has been all her life. When I asked her, "Do you believe in life after death?" she replied, "I often wonder about that. I think and I wonder if there really is. And I'm going to find out. I wouldn't bother worrying about it too much, because I'm going to find out." Rosemary pointed out that her feelings about death had changed greatly in later life.

"You know, when you're younger you go to bed and you think about death, and 'Oh my God!' Or you're sick: 'What if I don't wake up?' I don't think that anymore. Now that I'm old, I'm at peace when I go to bed. I figure if I don't wake up, well, maybe I'll be someplace nicer. It's just a funny thing. I used to be scared to go to sleep when I wasn't feeling good, but not anymore. I'm not ready to die or anything like that, but I'm just not afraid to die. I think there's something on the other side, and I've got some sisters over there who will be waiting for me. I'm not worried at all. And that's something I didn't think I'd ever come to terms with."

*All names have been changed.

From 30 Lessons for Living: Tried and True Advice From the Wisest Americans, by Karl Pillemer, Ph.D. Published by arrangement with Hudson Street Press, a member of Penguin Group (USA) Inc. ©2011 by Karl Pillemer.