Walking on sunshine: FitFlop creator Marcia Kilgore has designs on the rest of our bodies

A great deal of fashion's greatest successes are developed from a single idea: Diane von Furstenberg's wrap dresses, Calvin Klein's underpants, Yohji Yamamoto's black minimalism. Add to them Marcia Kilgore's FitFlops. She's the woman behind the cult shoes that have sold more than 4.8m pairs since launch two years ago, and which have just earned her the "Innovation of the Year" award from Harper's Bazaar magazine.

The astonishing success of the shoes would be enough for most moguls, but Kilgore is a creative polymath – and FitFlops are her third zeitgeist-defining invention.

Her first was Bliss in the 1990s, an at-the-time revolutionary day spa where treatments such as Triple Oxygen facials and Quadruple Thighpass anti-cellulite massages were carried out in dazzlingly white, comfortable rooms, while funky music pumped out and guests ate brownies and drank wine. Kilgore had the idea after she moved from her native Canada to New York aged 19. She taught herself to be a beauty therapist and, before long, Bliss was a favourite both with celebrities such as Madonna and hordes of hard-working New Yorkers. Bliss now has eight branches (seven in the US, one in London), but Kilgore, now 40, tired of the meetings and administration that goes with such a huge business and sold up.

So far, so entrepreneurial. The multi-million-dollar deal would be enough to see most of us sit back, count the money and eat violet creams. Especially if, as Kilgore was, heavily pregnant with a first child. But this dynamic expat, now decamped to London permanently with her husband Thierry (and sons Louis, now five, and Rafael, two), has a brain that won't stop churning out great ideas, no matter how unlikely the inspiration...

She picks up the story in her light-flooded Kensington office (all-white, natch). "The idea for a budget beauty range came from moving to England and seeing the celebrity magazines. I saw a picture of Kerry Katona with a circle around her sweaty armpit with the headline 'Celebrity disaster!'" she recalls with a hoot. "I thought, these tabloids are genius: an inexpensive laugh that takes you out of your mundane life. Why not mix that with the other thing that women love, cosmetics?"

And so the Soap & Glory range was born. The punning title is significant – humour is key to everything that Kilgore does. The catalogue for Bliss products had been chock-full of pithy descriptions and sparky suggestions. "Yeah. I got letters saying, 'I read the catalogue on the way to work and it made my day.'" So when it came to naming the S&G lotions and potions, Kilgore became what she laughingly calls "punmeister" again. Hence Clean On Me shower gel, Sexy Mother Pucker lip balm and Make Yourself Youthful face serum.

Cue more feedback. "I just got a letter saying, 'I'm a bearded old fart [Kilgore chuckles] and just wanted you to know that sometimes when I'm having a bad day I go into Boots and read your bottles to have a laugh.' He wasn't crazy either," she says. "He used to be in marketing and added that he wanted me to know he appreciated a good idea done well."

The ethos of Soap & Glory goes against all the perceived wisdom about beauty. You know, "here comes the science" and "85 per cent of the 171 women we asked said they preferred our conditioner" stuff... "All those products are making the same claims and sounding the same," Kilgore says. "Those statistics are meaningless. I just think, 'Would I buy this?'"

Women seem to love the fun approach. More than seven million S&G products have been sold globally since the range launched in 2006. It's due in no small part to the very competitive pricing and its availability in Boots stores in the UK, and in the budget chain Target in the US.

Janice Robson, head of exclusive brands at Boots, explains Soap & Glory's success: "The range has that unique positioning of being emotionally engaging (it makes you happy to use it), being accessible (both in terms of price and distribution), and, of course, having real credibility in the form of an expert behind it who has worked with us to deliver stunning formulations, underpinned with a great fragrance."

As Kilgore says herself on her packaging, it's for people who "don't want to spend an arm and a leg to moisturise one" and, agrees Robson, "who don't take themselves too seriously, but do want products that perform and make them feel good".

How did Kilgore know that global recession was around the corner – a time when a £200 face cream became a luxury beyond our reach and appetite? "It was good timing," Kilgore admits, "I'd heard the warnings about us all overspending, but I didn't consciously think, 'I'll do something inexpensive because financial armageddon is coming.' I just felt that the high end of the beauty market was not the direction to go in. There's too much there already."

One market that's not saturated is men's grooming. And so Soap & Glory For Men will hit the shelves next month – the bearded old fart will be happy. He's not alone; two of Kilgore's key stockists ran focus groups in which men lamented the lack of something inexpensive and fun for them; many were using their partner's Soap & Glory stuff in the shower. "There's a problem with male grooming at the moment," ' says Kilgore, who clearly makes a habit of quietly studying market conditions in between thinking up puns and designing funky footwear. "After years of growth, big companies like P&G [makers of such brands as Gillette and Olay] are reporting sales are way down. Maybe it's a recession thing? Maybe men out of work don't shave as often? Who knows!"

Even though we've been told that men are scruffing and moisturising like never before, the actual amount spent on beauty products by men is still, Kilgore acknowledges, very small. Which means Shave to Love, Soap'er Man and Dry Hard, priced at less than £4 each, should work. "I hope people get it," she says. "That this stuff is not taking itself too seriously. I hope men might buy it for their mates..."

There's another aspect of Soap & Glory products that goes against many of its competitors – it makes no claims to be "paraben-free". This buzz term refers to the chemicals that act as preservatives and are thought to have the potential to be absorbed into the skin and cause harm when built up.

Kilgore is disarmingly honest. "When I had my children, I used only organic products on them for, well, at least the first year. But I got sick of buying stuff that was separated and rancid. Now I use my own shower gel on them." She says that "only a tiny per cent" is absorbed when you use something on your skin or hair and "the skin is a pretty good barrier. I do use organic food because it's going in, and being digested, so you want it to be pesticide-free." It's a pick-and-mix approach to chemicals that's shared by many busy women without access to expensive organic skincare.

A product that will go down in history as having undisputed health benefits is the FitFlop. This thick-soled, jaunty-coloured sandal has changed the footwear landscape forever. It was launched in 2007 after Kilgore was on one of her fact-finding missions. At a cellulite seminar (yes, the beauty industry holds such events), she was musing, "Wouldn't it be great if there was a shoe that gave your legs a work-out while you walk?" – a real lightbulb moment. The FitFlop – whose patented Microwobbleboard midsole acts like a shock absorber and whose curved sole gently "floats" the foot – has legions of die-hard fans, who swear that their legs are more toned and slender as a result of wearing the shoes.

Kilgore is careful not to overstate the case for the FitFlop. "I can't say they do anything for cellulite because we haven't done any testing. But improved blood flow is a good thing, the foot can move properly as it spreads out and down and the body can start recalibrating itself." If the fan mail is anything to go by, FitFlops do all that and more. Kilgore has had to stop personally responding to letters, since they now number more than 20,000. There's the yoga teacher with a smashed leg after a car accident, who is now pain-free; the doctor with one leg shorter than the other, who has eased pressure on one foot thanks to the soft sole; and so on and so on. Kilgore had worked with Dr David Cook at London's South Bank University to develop the original sandals without any business plan for what they might become. A mention in a newspaper's "what's cool" column led to 55,000 hits on the FitFlop website "which was just a holding page, and the shoe they featured hadn't actually been made!" Kilgore remembers, laughing. She realised she was on to something with footwear that promised "a shoe with a built-in gym", "but you have to be cautious, because it could have just been a fad".

Not according to Omega Douglas, associate health director of Easy Living magazine. "I think FitFlops were such an instant hit because people have always loved the idea of shoes with health/body-improving functions – who wouldn't want to do nothing other than put on a pair of shoes and trot around for a bit to improve posture, leg tone and all the other things they claim to be able to do? But, other than adding a few inches to your height, previous toning training shoes never looked that great."

Indeed, as we know from clunky wooden Dr Scholl sandals and heavy, solid, MBT trainers, shoes that are good for you don't often look good too. And it explains why FitFlops have been declared as much a fashion success as a fitness one. Douglas again: "That's why FitFlops were such a breath of fresh air. They're comfortable, functional and don't look remotely offensive – especially in the summer. When everyone is padding around in sandals, you totally blend in and there's no risk of you looking like the Honey Monster!" Vogue, meanwhile, raved about last summer's gladiator style, and Liberty recently launched a limited-edition classic-print version that boasted a lengthy waiting list. Next Friday the winter collection goes on sale and is (no pun intended, sorry Marcia) a big step forward, style-wise.

There are metallic snow boots that scream hip-hop chic, and grey-felt clogs ("cool secretary", says Kilgore) that are cosy and comfortable. And shearling-lined long boots that make Uggs look, well, ugly. They all promise the same posture protection as the original "Walkstar" sandals.

Kilgore, ever the people's champion, keeps prices down too. FitFlops start at £36 and at £199, the knee-high Inuks compare favourably to boots which don't have the added bonus of toning your bottom. "We might design 20 styles and then when we test them, 17 drop out," she says. "We can't put something out there that doesn't feel right because, if they don't work, people will stop buying them."

There's no danger of that right now, as the nation's fashionistas breathlessly await the arrival of the new season's FitFlops. The current range has been spotted worn with Yohji Yamamoto minimal black, floral Marni dresses and everything in between. Could this be the end for ankle- snapping stilettos and weighty platforms? "A smart shoe that doesn't terrorise your body is absolutely the most fashionable," says Kilgore emphatically. "Low energy levels, an aching back... what's beautiful about that? I love men, but I'm not going to be uncomfortable for them."

So what's next for the queen of clean/princess of posture? FitFlops for children, next spring (the nation's parents sigh with joy as the tyranny of Crocs ends). Fun footwear that nurtures good balance and healthy bones is another sure-fire winner. And after that? People such as Newby Hands, health and beauty director of Harper's Bazaar, are watching and waiting. "Marcia's real talent is in creating products women connect with. While others rely on following trends and fulfilling marketing briefs, Marcia follows her gut, her head and her heart and it's this passion and creativity that makes everything she does so covetable for us and successful for her. In an industry choc-a-block with new products, I'm always interested in anything she's doing, because I know that given a few months it will be the one thing we all want." So...

"Well, both of my projects are still really interesting, and I don't do something new till I get bored," says Kilgore. "Plus I'm a working mother of two, and that's exhausting."

With that, she runs a brush through her signature sharp bob and bounds to her feet. It's difficult to believe there aren't at least a dozen more life-enhancing ideas percolating in her head. She's certainly not resting on her laurels or giving in to those violet creams. "I can't eat chocolate, because imagine if I got photographed and people said, 'Look at the FitFlop woman; she's got a huge ass.' Then it'd all go wrong."

For more information: www.fitflop.com. Soap & Glory is sold at Boots branches nationwide