It's not just love that makes a relationship work. The key to success lies with four cardinal rules.

We've all spotted them: those mysteriously happy couples swanning through the supermarket with a shopping list of ingredients for their cosy dinner party, their only argument whether to buy raddichio or rocket. And we've all wondered, what is their secret? In today's landscape of betrayal, when the average relationship lasts no longer than a sneezing fit, how are some people able to sustain and enjoy long-term relationships? Are they simply dullards who spend their weekends drinking tea in front of the telly? Is all that bliss a pose? Or are they doing something that the rest of us can learn from?

Perhaps they are. Mark Alperen, a psychotherapist who specializes in couples counselling, says that though the success of a relationship depends on your personal and family history, and that problems vary from couple to couple, over time he has seen certain issues that keep cropping up. Based on his experience, Alperen has arrived at four cardinal rules to long-term relationship success:

'First, there's the issue of respect,' he says. 'Allowing your partner enough space, having consideration for them. Second, it's important to allow the relationship to unfold at its own pace, instead of rushing into important decisions such as moving in together. A third important issue is trust. It's the foundation of a good relationship; couples must be committed to honesty. Finally, sex - you have to accept that it will change over time.

The experiences of women in long-term relationships seem to bear out Alperen's conclusions. While most cited 'being ready for something serious - and finding a man who is ready, too' as an important prerequisite, their ideas about what makes their relationships work echo Alperen's 'ardinal rules.'

Take Charlene Petrie who compiles music for film soundtracks. 'It's not a great idea to spend too much time inside your partner's head,' she says. 'Sometimes my boyfriend will be quiet, and I'll be convinced it's about us or me or something I did. I'll walk around for a few hours convinced that something's wrong. Then I'll finally ask and it will turn out to be some work-related issue that's making him moody. That sort of guesswork can be really unproductive.'

Charlene is one of those women who's always in a long-term relationship. In fact they've been so long that, at 30, she's only had time for two. When she met her boyfriend Andrew on a blind date, she had just got out of a ten-year relationship that started when she was 16. Andrew had never dated anyone for longer than a year.

'After a few dates,' she recalls, 'our relationship took off like wildfire - well, a quiet wildfire. But we didn't move in together until we'd been together for more than a year. Sure, there are exceptions, but I think that nothing can kill a love affair quicker than moving in too soon.'

'There are a lot of stages that you go through in the first year of a relationship - at three months, at six months, at 12 months,' Alperen says. 'Changes happen that you can't always anticipate in the first bloom of romance. It's best to be patient. Slow and steady wins the race.'

Nicole Ramirez and Leo Layner are another couple that are taking it slow. Nicole, a 33-year-old jeweller, recently became engaged to Leo, a 38-year-old web designer, after living together for nearly six years.

'Before I met Leo, I had a two-year limit on my relationships,' she says. 'And Leo - well, before he met me, let's just say he was a very bad boy!' But this time around it's different, Nicole explains: 'When Leo and I got together we agreed to be really honest, to say exactly what was on our minds. That keeps the relationship fresh because we don't have any secrets or bad feelings weighing us down. Everything stays as light and fresh as it was when we first got together.'

'It might not always be easy to cultivate the trust of your partner,' says Alperen, 'but if you're truly honest with him, you feel known, and that's the only way you can really feel loved.'

At first, the commitment to being honest wasn't easy for Leo, who says he had lied to past girlfriends about fidelity. But, he says, Nicole's willingness to allow him plenty of time to himself ironically has kept him from feeling the need to stray or to create secrets between them.

'When things get too intense, we spend a night or two apart,' he says. 'Sometimes I just need to get away for the weekend with the boys. Nicole doesn't mind at all; in fact, she understands. When we finally get together again, it's like, 'God, I missed you'. We're like sex maniacs!'

Which brings us to one of the most oft-cited stumbling blocks to long-term relationships: sex. After those first heart-thudding months, sex rarely retains the excitement of the new. In some instances, with a deepening comfort level, sex intensifies; in others it fizzles.

Instead of panicking at the first signs that passion's lost its edge, long-term couples must trust that with time and perhaps a little effort, their physical relationship can improve. 'If there's something negative going on in the relationship it will be reflected in a couple's intimacy,' remarks Alperen. 'Some couples think their problem is sex, but usually it's something else: either poor communication, or perhaps a lack of trust, or one partner wanting to dominate the other. They have to be committed to working on the problem instead of fleeing at the first sign of trouble.'

This willingness to tackle difficulties is what saved Jaye Teplow's marriage to her husband Darius. 'There was one point during our marriage when the spark completely went out of our sex life,' says Jaye. 'I was too involved in a family crisis, Darius was working all the time and had stopped confiding in me and sharing those day-to-day details. In terms of sex, our relationship felt dead.'

Instead of letting the problem drift until it was too late, Jaye says that she and Darius made an effort to begin communicating more, and over time their sex life improved. 'It's always going up and down,' says Jaye. 'One year, we'll both have other things on our minds. But the next year we'll rediscover each other erotically. And if one of us is unhappy, we talk about it.'

Alperen agrees with this strategy: 'Most married people argue at some point about sex or money. The best solution to talk about issues as they arise, instead of clamming up and allowing them to fester.'

While most psychotherapists would prescribe the talking cure, some couples prefer the giggling one. 'It sounds corny, but we laugh a lot, says Kristin Berneys, a 37-year-old fashion stylist who has been with Fred, a sports marketing director, for nearly a decade. 'Sometimes our relationship is heaven, sometimes it's crap. But even if the world was falling apart, Fred would have me in stitches. If we didn't laugh so much, we'd probably have killed each other long ago.'