She turns 50 in a couple of months, and while those cheekbones betray some nip-tuck action, Madonna's stunning vital statistics are all her own work. In the 25 years since she first hit the charts, the superstar has sold over 200 million units to become history's most successful female musician. Her personal fortune stands in excess of €250 million.

And there is no end in sight to the blonde ambition of the woman who famously "pulled herself up by her bra-straps" and who was recently inducted into America's Rock'n'Roll Hall Of Fame. This month she releases Hard Candy, her 11th and final studio album for Warner Brothers, the label she signed to in 1982.

It is exceedingly rare for any label to stable an artist for 26 years. Most acts are cruelly dumped once sales start to falter. Madonna has never had that problem. When she suffered the worst dip of her career, with 2003's American Life, it still shifted five million copies. Her last Warners release, 2005's Confessions On A Dance Floor, shipped a phenomenal eight million copies at a time when CD sales were plummeting in the face of competition from the download market.

Some years back, Madonna refused to allow her songs to sell on the iTunes online store. She has corrected that shortsightedness, and the individual tracks from this month's Hard Candy will start raking in revenue as limited edition mobile phone downloads, before the full album is available.

Madonna's imminent departure from Warners underlines the keen eye for the bottom line which has always been her hallmark. Twenty years ago, acts embarked on tours that might only break even, or even lose money, in order to promote money-spinning albums. That situation today has turned on its head. Today, new albums are tools to promote live shows which make big money through fat ticket prices and the sale of t-shirts and other memorabilia.

Madonna is one of the biggest live draws on the planet. Her 2006 Confessions Tour was the biggest earner by any female act, playing to 1.2 million people and grossing €200 million.

To accommodate the revenue shift between touring, recording and merchandising, new players have emerged with new business models. The most significant new player is the US firm Live Nation, which offers to synchronise the commercial activities of top acts in an all-in-one package.

Live Nation has been credited with boosting gig attendances in the US by 13 per cent last year, and turning loss-making tours into profitable ones. This week, Live Nation signed up U2, but the first act the company targeted, and landed, was Madonna. She recently agreed a 10-year deal with the one-stop-shop which will earn her a rumoured €100 million and keep her in gainful employment up to the age of 60.

U2's impressive longevity at the top of the pop tree is easy to fathom. The Dublin band fit neatly into the decades-old tradition of reliable stadium rockers, like The Rolling Stones, Led Zeppelin and The Who. With those bands, and their frothy lite placebos like Bon Jovi, you pay your money knowing that you're going to get more or less what you got the last time, and each time before that.

Madonna's enduring mass appeal is more complex, because unlike the reliable old rockers she has a long track record of confounding the expectations of her public. The two recurring themes running through her work, and the soap-opera of her life, are sex and religion, but knowing the basic ingredients doesn't tell you what she's going to cook up with them.

One of her hit song titles set the poser Who's That Girl? and she's always taken pains to keep her audience guessing. We've seen her as the teen style-icon, the baby-doll Boy Toy, the breathless bondage queen, the devoted mother, the butter-wouldn't-melt matinee star, the tacky Z-movie porn siren, the author of children's bedtime stories, the full frontal nude, the loving wife, the vindictive diva, and the gyrating seductress of a black plaster saint (in the allegedly blasphemous Like A Prayer video).

Through all the twists and turns, she has always displayed an almost miraculous instinct for tapping the zeitgeist. Early on, she mixed-and-matched different aspects of New York street chic (lace tops, skirts over pants, fishnet stockings, clunky jewellery) and marketed the look to teenage girls around the globe as her own.

Acutely attuned to the possibilities of the newly arrived pop video, she got her foot in the door from the beginning. When MTV was new, with hours of schedules to fill but few videos, Madonna was playing on a non-stop loop with colourful fantasies such as Material Girl and Like A Virgin. At the very first MTV Video Music Awards, her sexually charged live performance of Material Girl made her the talk of the town.

A winning formula of mixing showbiz with mild outrage had been set, which endures to the present day and the release of Hard Candy. Sharing its title with a notorious 2005 movie about paedophilia, Hard Candy has already attracted its share of the type of concerned-parent publicity that money can't buy.

Madonna's instincts were so finely attuned that within a few years of breaking from obscurity, she was a credible rival to Princess Diana for the title of the World's Most Famous Woman. The contrast between the pair ran deeper than the fact that one was cast as the Fairy Princess and the other as the Scarlet Woman. Diana's fame was the creation of others (the British Royal family) in the service of others (to provide an heir and a spare). Madonna's success, on the other hand, has always been created entirely by Madonna to serve Madonna's every need and whim.

Her endless faith in her own intuition was apparent from the start. Madonna hand-picked dance producer Reggie Lucas to make her first album. However, with a few songs recorded, she decided he was the wrong choice. Despite the gulf in seniority and experience, she removed the tracks from Lucas and handed them for completion to her boyfriend, John 'Jellybean' Benitez.

Some years later, Madonna released the single Vogue, with a striking video featuring her in a series of poses associated with Greta Garbo, Marilyn Monroe, Marlene Deitrich, Jean Harlow and others. The viewer never thinks for a second that Madonna sees herself as standing on the shoulders of these Hollywood giants. Instead, she is cannibalising their images in the service of her own.

While she has no apologies to make for being so deeply self-consumed, she does offer an explanation, which goes back to the death of her mother from breast cancer at the age of 30 in 1963. She revealed: "The greatest event in my life was my mother dying. What happened when I was only six years old changed forever who I am. I can't describe in words the effect it had.

"That's when the die was cast. I know if I'd had a mother I'd be very different. Her death gave me a lot of what are traditionally looked upon as masculine traits in terms of my ambitiousness and aggressiveness."

But there are other traits beyond masculine ambition and aggression which explain why Madonna has just signed up for another 10 years on the treadmill when she could be enjoying quality time with the kids.

With countless millions already in the bank, she reflected some time ago: "I was raised by a working-class father and we never had money. I feel sometimes that someone will come and take it all away from me. That makes me work really hard all the time."

- Damian Corless