How To Stay Married was Jilly's first book, written long before she became known as 'queen of the bonkbuster'. It wasn't as enduring as Riders or The Man Who Made Husbands Jealous, but it transpires that the author knew a thing or two about the staying power of a marriage: the book was re-released in 2011 to coincide with the couple's golden anniversary.
Jilly penned a new foreword for the book, written from a 50-year perspective, but first she re-read How To Stay Married - and "nearly died of horror".
"What a smug, opinionated, proselytising little know-it-all I was back then," she wrote. "How could I have insisted that 'a woman should be grateful her husband wants her', and suggested that if a wife refuses her husband sex more than two days running, then she has only herself to blame if he's unfaithful."
The author laid down the law in How To Stay Married but, 50 years later, she admitted that she didn't always practice what she preached.
She advocated "total honesty about money", yet she regularly smuggled new clothes into her wardrobe. She said no wife had the right to "go to seed" in the first edition, only to admit that she becomes "a positive hayfield" when she's approaching a deadline.
And before you ask, she regrets that there "wasn't a word for husbands on how to exert self-control to avoid a beer belly".
As the author says herself, How To Stay Married was written in a different age.
"No one had dreamt up 'new men' or paternity leave, and two-career marriages were a rarity, particularly if the couple had children," she explained.
How To Stay Married, like any 50-year-old guidebook, is old-fashioned, unapologetically sexist and, at times, downright offensive. But it's also warm, witty and - whisper it - wise.
While a lot of the advice now seems anachronistic, it's worth remembering that this book was progressive for its day. Jilly urged would-be Stepford Wives to use a launderette if they could afford it, just as she suggested that transcendent sex was more important than a ship-shape home.
The author's wedding advice is as relevant now as it was then. "Dope yourself with tranquillisers by all means," she advised brides-to-be, "but watch the champagne later." And don't worry if you get bored on honeymoon "when you have nothing to do for a fortnight, except each other".
Her classification of men with a wandering eye has also stood the test of time. According to Jilly, they are either 'open gazers' or 'secret doers'. The former is "just testing his sex appeal like gorillas beat their chest". The latter takes the next, high-risk, step.
Her own husband had a very public affair in the Nineties. The couple survived the transgression and were devoted to one another until Leo's death in 2013. Nonetheless, Jilly cut out some of the more "hubristic" advice on infidelity from the second edition of the book.
"We've had marvellous patches, and patches so bad that they rocked our marriage to its foundations," she wrote in the forward for the new edition. "But I've come to realise that if you can cling on like a barnacle during the bad patches, your marriage will survive and, in all probability, be strengthened."
Most married people will agree with this particular piece of advice, but what about the wisdom she imparted way back when? Is it still relevant today? Let's take a look…
It's important to remember that Jilly's first book was written long before Nancy Friday, Ann Summers and Carrie Bradshaw came on the scene. Male pleasure took an unfair precedence in 1969 and there was no Twitter mob ready to take the author to task for writing: "If you amuse a man in bed, he's less likely to bother about the mountain of dust underneath it."
Jilly was absolutely right, though. Sexually satisfied men are much more forgiving. She just failed to mention that sexually satisfied women are too.
"In any good marriage," wrote Jilly, "sex should get better as the years go by, even if you indulge in it less often."
Sage advice, but it's worth remembering that it was penned before online pornography, sleep deprivation and stress-related low libido became the banes of our sex lives.
… And more sex
"The first essential is to be honest," wrote Jilly. "Don't pretend to be in ecstasies of excitement if you are not, or your partner will assume he is doing the right things to please you."
It's hard to argue with this advice, especially since most of us received our formative sex education from Jilly's books.
The division of labour
The curious thing about How To Stay Married is that its author can go from being thoroughly modern to wincingly backward in the space of a single chapter. "If the wife is working, the husband must be prepared to give her a hand," she opines. "Equally, it's up to the wife to say when she needs help."
This seems fair and progressive, until she asserts that "men detest seeing women slaving in the house" and suggests that working women arrange to work from 8.30am to 4.30pm so that they can rush home to clean, iron and cook before their dear husband arrives home. No wonder Valium became known as Mother's Little Helper during the Sixties…
Nothing has changed in the 50 years since Jilly opined that in-laws can be troublesome. "As my mother-in-law once pointed out to me," she wrote, "nobody is ever good enough to marry one's children."
However, her suggestion for softening them up is probably best avoided in the #MeToo era. "The husband's best tack is to flirt with his mother-in-law," she suggested. Really?
"Tell your husband when he looks handsome," wrote the author. "And even if you are the type of man who can't tell a discarded false eyelash from a centipede, compliment your wife on her outfit."
It's hard to argue with this advice almost 50 years later.
Flattery will get you everywhere, now, just as it did then. And yes, a large proportion of men still have no idea what those spindly things on the bedside table are.
How to Stay Married offers lots of practical pointers for detecting infidelity - for both husbands and wives. Men are told to be wary if their wife "doesn't look dismayed when you say you're going to America for three weeks". Women are advised to raise suspicion if their husband "looks happy on a Monday morning and miserable on a Friday night".
Still, it might be helpful if Jilly didn't lay the blame for a husband's infidelity squarely in the lap of his wife. She said men with moody, demanding or frigid wives are more likely to stray, but nowhere in the book does she look at the male shortcomings that might make a wife cheat.
"One of the great myths of marriage - heavily fostered by television commercials featuring smiling young couples up ladders - is that home decorating is fun when you do it together.
"It isn't," quipped Jilly. "It's paralysingly boring and caused more rows in our marriage than anything else."
This advice was penned long before the rise of flatpack furniture, which just goes to show that some things never change.
Jilly has some timeless advice for the bone of contention that is Christmas Day. Instead of arguing over whose parents you're going to spend Christmas Day with every year, she suggests that couples "get a large dog and say you can't leave it." Bingo.
A wicked sense of humour is as important as wild sex for Jilly. The sign of a healthy marriage, she writes in the new edition of the book, is "creaking bed springs - as much as from helpless laughter as sex".
She also believes that humour is the best way to defuse a row. "My husband once, mid-row, put both feet into one leg of his underpants and fell over. I went into peals of laughter and the row was at an end."
It's a sweet vignette that suggests the author always has a wry take on a situation, and it makes it that little bit easier to take How To Stay Married with a pinch of salt.