Rhiannon Harries: Madge, it's time to stop changing before you forget who you are

If necessity is the mother of invention, what is it that begets reinvention? Every time I clap eyes on a celebrity's "amazing new look" on the cover of anything from OK! to Vogue, I'm tempted to conclude that, when it comes to self-reinvention, insecurity and desperation are primary motivators.

For a long time, periodic metamorphosis has been seen as the trademark of the canniest stars. Madonna and Kylie were the doyennes of the image-and-sound overhaul. What was impressive was never so much the changes per se, as the way in which they anticipated (with the help of an army of trend scouts, no doubt) new undercurrents in fashion and music.

But with the Queen and Princess of Pop now in their 50s and 40s respectively, there is something a bit sad about the endless makeovers. Tuning into what appeared to be a dedicated Michael Jackson music channel the other day, I was struck by how consistent his image was by comparison – from the Thriller days to his final public outings.

Jackson was, of course, no stranger to reinvention; few could begin and end their life with so little in common between their physical appearances. But aesthetically, he had a single vision of who he wanted to be and he stuck to it – and there is something impressive in that commitment.

Flicking channels, I caught Jarvis Cocker, Jackson's one-time Brit Awards nemesis, in the 1995 video for Pulp's "Common People". In his charity-shop suit and skinny tie, Cocker looked pretty much identical to when I saw him play a few weeks ago and, because of that, the video felt less dated than it might have.

By contrast, Madonna and Minogue, and the gaggle of D-listers who have picked up their shape-shifting tricks, look less like postmodern mistresses of their own identity than publicity-seekers.

There is unfair pressure, of course, on female stars to keep mixing things up, but there are only so many times you can play the reinvention card before your multiple personas add up to no persona at all.

In this instance – and only this – it's worth looking to Mrs Beckham for inspiration. Try as she might to turn herself into an edgy fashion plate, Posh can never shrug off her real self. The creosote tan, thigh-skimming hems and matchy accessories haven't changed since her "Wannabe" days – they've just got more expensive.

"Knowing who you are and being it like mad" is the best advice, style or otherwise, I've heard. If that doesn't coincide with the latest vogue, so much the better – you can't go out of fashion if you've never been in.