"After three hours sleep I feel like an old bag! x vb#NYFW" Beckham tweeted with a picture of herself with a large purple tote over her head.

Fact is, VB's bags are far from the brown paper grocery store variety. They all have four-digit price tags and are crafted from luxe leathers such as buffalo, calfskin and camel. The super-luxe Alligator Victoria bag had a price tag of £18,000, or around $A27,800, when it was released last year.

The path to celebrities-as-fashion designers is peppered with roadkill.

Lindsay Lohan's ill-fated design career ended after a single collection for Parisian label Emanuel Ungaro in 2009 and when Kanye West's unveiled his debut fashion range at Paris Fashion Week in 2011, one wag described it as "rap with a capital-C."

When Beckham made her NYFW debut as a designer in 2008, she appeared to be yet another celebrity cashing in on her profile with a collection she probably would pay someone else to design for her and perhaps half-heartedly promote a couple of times a year.

Fact is, Sunday's picture of Beckham with the bag over her head shows her working in her studio "after three hours sleep," albeit in six inch stilettos dripping with diamonds and a stonking great gold wristwatch as her manicured fingers tap over the computer keyboard.

You may be shocked to hear this, but Beckham actually does much of the grunt work herself. She has shed the French manicures and orange spray tans of her wag era incarnation and is now wholly focused on her career in fashion. Her earnings are not yet outstripping hubby's, but sales of Beckham's two ready-to-wear lines, sunglasses and denim totalled £60 million in 2011. That's hardly a designer dalliance.

Beckham's debut creations received rave reviews from hard-nosed fashion editors, even though they came with a little inspiration from Roland Mouret, whose figure-enhancing dresses were firm favourites of Beckham's before she began her own design career. But over time the former Posh Spice has honed her own design signature into unashamedly posh dressing, including elegant, calf length dresses and pencil skirts, figure-flattering peplum tops and of course, the rather pricey handbags. Her fashion show on Sunday at the New York Public Library also received an overwhelmingly positive response for its focus on more youthful and softer silhouettes, which for the first time included trousers, in addition to Beckham's ubiquitous dresses.

Unlike many other celebrity "designers", Beckham has been able to build such a credible profile in the fashion industry because she takes it seriously.

When she first showed at NYFW, Beckham personally talked editors in the front row through every single look on the runway, explaining her design process and the rationale for why each garment was a certain way.

Along with investing her own time in the brand, Beckham has recruited a strong sales team and design department and has been honest enough to admit repeatedly that she is still learning about the fashion business.

Another factor in her success is that she actually wears the clothes. While some other celebrities put their names to collections yet rarely - or never - wear them, Beckham is photographed in her own outfits on an almost daily basis, and in terms of her polished, classic style, is her own best ambassador for them.

"The bottom line is, if it's not right for my wardrobe, then it's not right for the brand," she told London's Guardian newspaper in February.

In this sense she is similar to American twins Ashley and Mary-Kate Olsen, who through a laser fashion focus and hard work have made the transition from child stars into the businesswomen behind several highly successful clothing lines, including luxury brand the row, which earlier this year snared them the Womenswear Designers of the Year award at the prestigious Council of Fashion Designers of America awards.

Beckham won the equivalent Designer of the Year gong at the annual British Fashion Awards last year, and the two labels also share a penchant for posh purses: The Row last released a $US34,000 alligator backpack that - gasp! - not only sold, but sold out almost overnight.

But despite Beckham and the Olsens professing their business growth is mostly due to hard work, of course it's also due in a big way to their celebrity, and they remain remarkably high maintenance despite their protestations their success is sustained by a strict diet of humble pie. For example, Beckham is rarely seen in anything but towering Louboutins - although that may change because she - shock alert! - showed flat gladiator sandals during her NYFW show - and glamorous gowns surrounded by a hefty security detail emerging from a blacked-out limousine.

It's just that these days she's wearing her own clothes when she does so, and many others are wearing them too.