In the smart restaurant of a very smart hotel in the West End of London, Roy F Baumeister, eminent American social psychology professor, orders a lunch of fish and chips, and then decides not to eat the chips. "I won't eat something that's not good for me unless it's absolutely perfect, and it's going to give me real pleasure," he says. "I'm afraid ... Well, it just didn't look like these were going to do either."

What willpower, you might say. You'd be right; the chips looked pretty good. But Baumeister is also, coincidentally, a leading authority on that very subject, and has just published a smash-hit book on it with New York Times science writer John Tierney.

Willpower: Rediscovering Our Greatest Strength distills three decades of academic research (Baumeister's contribution) into self-control and willpower, which the Florida State University social psychologist bluntly identifies as "the key to success and a happy life".

The result is also (Tierney's contribution) readable, accessible and practical. It's an unusual self-help book, in fact, in that it offers not just advice, tips and insights to help develop, conserve and boost willpower, but grounds them in some science.

Willpower is, Baumeister argues over lunch, "what separates us from the animals. It's the capacity to restrain our impulses, resist temptation – do what's right and good for us in the long run, not what we want to do right now. It's central, in fact, to civilisation."

The disciplined and dutiful Victorians, all stiff upper lip and lashings of moral fibre, had willpower in spades; as, sadly, did the Nazis, who referred to their evil adventure as the "triumph of will". In the 60s we thought otherwise: let it all hang out; if it feels good, do it; I'm OK, you're OK.

But without willpower, it seems, we're actually rarely OK. In the 60s a sociologist called Walter Mischel was interested in how young children resist instant gratification; he offered them the choice of a marshmallow now, or two if they could wait 15 minutes. Years later, he tracked some of the kids down, and made a startling discovery.

Mischel's findings have recently been confirmed by a remarkable long-term study in New Zealand, concluded in 2010. For 32 years, starting at birth, a team of international researchers tracked 1,000 people, rating their observed and reported self-control and willpower in a different ways.

What they found was that, even taking into account differences of intelligence, race and social class, those with high self-control – those who, in Mischel's experiment, held out for two marshmallows later – grew into healthier, happier and wealthier adults.

Those with low willpower, the study discovered, fared less well academically. They were more likely to be in low-paying jobs with few savings, to be overweight, to have drug or alcohol problems, and to have difficulty maintaining stable relationships (many were single parents). They were also nearly four times more likely to have a criminal conviction. "Willpower," concludes Baumeister, "is one of the most important predictors of success in life."

So how can we improve ours? Baumeister's big idea, now borne out by hundreds of ingenious experiments in his and other social psychologists' labs, is that willpower – the force by which we control and manage our thoughts, impulses and emotions and which helps us persevere with difficult tasks – is actually rather like a kind of moral muscle.

Like a muscle, it can get tired if you overuse it. Exercising willpower, but also making decisions and choices and taking initiatives, all seem to draw on the same well of energy, Baumeister has established. In experiments, he found that straight after accomplishing a task that required them to restrain their impulses (saying no to chocolate biscuits, suppressing their emotions while watching a three-tissue weepy), students were far more likely to underperform at other willpower-related jobs such as squeezing a handgrip or solving a difficult puzzle.

"The immune system also dips into the same pot, which is big, but finite," says Baumeister, "and, we are pretty sure, so does women's premenstrual syndrome. Having a cold tends to reduce your self-control, and PMS does the same. We get cranky and irritable, but it's not that we have nastier impulses – it's that our usual restraints have become weakened."

So best avoid trying to do too many things involving mental effort at the same time, or if you're ill. As with a muscle, though, you can train your willpower. Even small, day-to-day acts of willpower such as maintaining good posture, speaking in complete sentences or using a computer mouse with the other hand, can pay off by reinforcing longer-term self-control in completely unrelated activities, Baumeister has found. People previously told to sit or stand up straight whenever they remembered later performed much better in lab willpower tests.

The final way in which willpower resembles a mental "muscle" is that when its strength is depleted, it can be revived with glucose. Getting a decent night's sleep and eating well – good, slow-burning fuel – is important in the exercise of willpower, but in times of dire need a quick shot of sugar can, according to Baumeister's lab tests, make all the difference.

(This is, of course, something of a problem for crash dieters, who basically need to eat in order to summon up the willpower not to eat. Indeed some very strong impulses, such as the behaviour often exhibited by males in possession of an erect penis, can sometimes prove completely resistant to willpower, even after the ingestion of a can of Coca-Cola.)

Baumeister cites a "very impressive demonstration" of the glucose argument: in a study published last year, researchers found that Israeli judges making the difficult and sensitive decision of whether or not to grant parole opted to do so in roughly 65% of cases after lunch, and hardly ever just before.

Baumeister's top willpower tips: Build up your self-control by exercising it regularly in small ways. Learn to recognise signs that your willpower may be waning. Don't crash diet. Don't try to do too much at once. Establish good habits and routines that will take the strain off your willpower. Learn how to draw up an effective to-do list.

Don't put yourself in temptation's way, or if you can't avoid it, make it harder for yourself to succumb. Use your willpower actively: plan, commit, and do so (like members of religious communities) publicly. "People with low willpower," Baumeister says, "use it to get themselves out of crises. People with high willpower use it not to get themselves into crises."

Much of this, of course, is in the book. You may even learn how to say no to chips.