Last week it was announced that the two-year collaboration between Ant and Dec and Morrisons is coming to an end. National treasures aligned to the supermarket's northern heritage; their union likely looked good on paper. But without any real food credentials, Ant and Dec failed to build quality fresh food perceptions that Morrisons is trying to create.
Yet in the commonplace world of celebrity endorsements, not all are expensive blunders. Many are hugely successful. George Clooney and Nespresso. Michael Jordan and Nike.

What is the formula for the perfect pairing? And how are these relationships changing?

Why use celebrities?

Endorsements allow brands to "borrow' some of the celebrity for themselves ’ whether it's a lifestyle, attitude or fan base. Rosie Huntington-Whiteley made M&S lingerie sexy to younger demographics. Patrick Stewart made Nintendo DS a credible choice for grown ups. It's a quick way to demonstrate personality in a crowded market.

This approach still exists ’ just wait for the curious combination of Sly Stallone and Warburton's loaves out later this year. But it's increasingly being replaced by smarter partnerships.

Technology and social media are changing the world of celebrity, redefining who is and isn't influential and changing what we know about them.

The changing landscape of celebritydom

The Internet has democratised celebrity status. Across Twitter, Instagram and YouTube, near-unknown bloggers and vloggers rapidly become as influential as professionals. Celebrities have never been more human - we know what they eat, think and discuss at their kitchen table ’ making their relationships with retailers more realistic than ever.

But consumers are no longer naA?ve; they know more about each celebrity, so are quick to see through disingenuous partnerships. Appointing Alicia Keys, a well-known Apple user, as Creative Director of Blackberry was never likely to be a success.

So what makes a perfect pairing?

The celebrity needs to address a brand issue

Partnerships are most effective when the celebrity is hired to "solve' a specific brand issue.

Jamie Oliver offset Sainsbury's reputation of being "too elitist, too upmarket and a little too arrogant'. Young, affluent ’ but easy to identify with ’ Jamie helped the supermarket reconnect with customers over the joys of simple, great-tasting food. According to Sainsbury's, in 2001, he directly contributed A?153m out of A?535m profit. Pretty pukka, by anyone's standards.

The partnership needs to feel authentic

Gary Lineker brought fun to Walkers crisps; Thierry Henry brought a certain "va-va-voom" to the Renault Clio. Both work because the celebrity's background matches the brands' heritage. Even more than this, they feel authentic because they're plausible: Gary Lineker could have Walkers crisps in his kitchen cupboard; Thierry Henry might really drive a Renault Clio.

The relationship needs to be mutually beneficial

A pretty face fronting a campaign is all fine and well. But the most successful partnerships are those where both brands stand to gain something.

Kate Moss and Topshop: a perfect marriage still going strong today. Women across the world can buy a little bit of the supermodel ’ her fashion and, by extension, her lifestyle. The collaboration facilitated Topshop's global expansion and made the brand appealing to older, wealthier customers, increasing sales by 10%. Moss benefitted from the partnership too, bouncing back from the drug allegations that lost her A?8m worth of contracts. In doing so, she once again became one of the world's most sought-after models.

Relationships turned sour

Endorsements are still risky business. You can't control a celebrity ’ or their reputation. Look at Lance Armstrong and Nike. Or Tiger Woods and P&G.

The perfect formula

Brands need to choose a celebrity with an affinity to their brand, and present them with a mutually beneficial proposition that stretches beyond sheer financial incentive. Someone with something the brand doesn't have, that can change perceptions for the better.

And, when a growing body of consumers are hyper-connected, media-savvy millennials, choosing a social media hero with widespread influence is an increasingly smart move.

But how to ensure your celebrity of choice actually wants to be associated with you? Then brands need to strengthen their own position, using celebrity endorsements to go the extra mile, rather than doing all the hard work for them.

And as for Morrisons? They need to put the endorsements on hold until their fresh food credentials speak for themselves.

Rosalind Bull is a Consultant at The Clearing.