I've had the same best friend since the first grade, although we haven't lived in the same state since the Clinton administration, and we almost never call when we say we will. Without the obvious trappings of a good friendship, such as face time and basic thoughtfulness, how can we possibly feel so close? As I learned from talking to experts, those conventional friendly actions have much less to do with how satisfying a friendship is than with what you give each other on a more nuanced, psychological level. Here are some ways to strengthen your bonds—no sentimental cards required.

#1 Share a secret. It's no surprise that telling a friend something sensitive would make her feel like you trust and value her. But that's not the only reason it brings you closer. It also makes your friend feel as if you value her more than someone else, says Peter DeScioli, a postdoctoral fellow in psychology at Harvard University, who has done extensive research on the “alliance” theory of friendship. He has found that even if you don't score someone very highly on likable traits such as intelligence or kindness, you are more likely to rank her high up in your friend group if you know she counts you among her closest friends. This ranking idea sounds petty, but DeScioli explains that “we want our friends to take our side, even if it's a minor disagreement about where we all want to go out to dinner tonight.” So dish out that gossip—and your friend will know you've got her back.

#2 Treat him like a grown-up. When a pal is struggling, it can be awfully tempting to grab him by the scruff of his neck and just tell him exactly what he should do. After all, isn't giving good advice part of being a good friend? Perhaps not, according to researchers who study self-determination. Edward Deci, a psychology professor at the University of Rochester, has found that supporting a friend's autonomy—that is, making him feel as if he can make his own choices—creates a better relationship and may even improve his mental health. In one study, Deci and his colleagues did in-depth interviews with pairs of friends and found that the more of this type of support there was from a friend, the more satisfied the partner was with the friendship and the higher self-esteem the person had. “When people are relating to you and acknowledging your sense of importance, your sense of competence, you feel better about yourself,” Deci adds. In other words, treating a friend like he's got his act together could actually help him get there.

#3 Accept that no friendship is perfect. Everyone readily admits that fights are a normal part of marriage and family relationships. But when it comes to friendships, people are much less comfortable accepting imperfections. This tendency can cause friends to give up too fast on their relationship when trouble arises, according to Jan Yager, a sociologist and author of the book Friendshifts: The Power of Friendship and How It Shapes Our Lives (Hannacroix Creek Press, 1999). “People have this mistaken notion that a friendship is supposed to be the respite from all the other challenges in one's life,” Yager says. “You have to deal with the co-worker or the boss, you have to deal with your relatives, but you choose your friends.” Don't be afraid of the occasional tiff or misunderstanding. Once you make it through, the bonds are likely to be stronger than ever.

#4 Be there—physically—during hard times. This summer I'll fly down to California to spend a few days helping my best friend after her third child is born. It might be the only time I see her in 2013, but in terms of keeping our friendship strong, it'll probably be worth more than a dozen just-for-fun visits. Having a friend standing by can actually blunt the body's stress response during a trying experience, according to a study by William Bukowski, a psychologist at Concordia University. He and his colleagues asked schoolchildren to keep a journal of personal interactions for four days, and they took several saliva samples daily to measure levels of the stress hormone cortisol. They found that cortisol levels were significantly lower in kids who said a close friend was with them during a negative event, such as getting in trouble with a teacher. “Our bodies recognize that they don't have to respond so much to danger if someone is there to help,” Bukowski says.