William and Kate's relationship provides some useful tips on a successful marriage

On Friday, the world will watch an ordinary girl from the Home Counties become a princess when Catherine Middleton marries Prince William at Westminster Abbey.

There can be no doubt this is a genuine love match – unlike the marriage of William’s parents nearly 30 years ago.

As well as being a love story, the relationship of William and Kate is also a partnership of equals, as the historian David Starkey explained in a TV documentary.

In an accompanying newspaper article, he said: “Diana was like a frightened rabbit, she was totally unprepared for public life,” whereas Kate Middleton “is an educated woman. She’s William’s equal and in fact she comes across as more confident and poised than him.”

Being an equal of your partner – regardless of your respective social status – is a key element in a successful relationship. One reason for this is that there is more likely to be an equal balance of power within the marriage.

A few years ago, I wrote on this blog site that if a relationship becomes unbalanced, it can damage the emotional and psychological connection between a couple.

However well-matched a relationship may be in the early days, it can become unbalanced as time goes on. There are all sorts of reasons for this: one partner might fall ill and need looking after; one half of the couple may lose their job and feel inadequate if the other is flying high, career-wise.

As I wrote previously, a shift in power – however accidental – is one of the most common triggers for divorce and it’s not hard to understand why. The high-octane party starts to get bored with or even despise their weaker partner, while the least powerful resents or is in unhealthy thrall to the stronger spouse.

If you feel your relationship is suffering because the power balance has shifted uncomfortably, it is a good idea to address the problem sooner rather than later. For example, if you’ve become a stay-at-home mum with a husband who’s a jet-setting executive, find ways to compensate for this: become a school governor, get involved with charity work or study for an Open University degree.

Respect for your spouse is almost as important as love. So, too, is friendship. And what Kate and William undoubtedly have is a rock-solid friendship. The Prince said in an interview that he and his soon-to-be princess started out as “just friends” before the relationship developed into a romance. Add to that the fact they’ve been together more than eight years and what you get is a well-rooted, carefully-considered and workable relationship.

Again, then; if you fear your relationship lacks the passion it once had, think about whether your partner is your best friend. If the answer is yes, then count yourself lucky. Companionship, in the long term, is of greater value than sexual frisson.

So, on Friday, as we watch William and Kate exchange their vows, let us take from their love story some useful tips on how successful relationships are forged – and are built to last.