Hiring a personal trainer is worth a dozen barely used gym memberships, not least if your willpower is as poorly defined as your abs. But as well as working your glutes, quads and wallet, a partnership with a PT is just as much an exercise in diplomacy, deference, confidence and honesty. Good results come to those who do their research and bring a good level of industry and self-awareness to the bench. You have to know what you want and how to ask for it.

“In some ways, it’s quite simple – I take payment and you’re on time – but outside of that, it depends on so much,” says Mr Nick Finney, a leading London trainer whose clients have included Mr Robbie Williams and Ms Jennifer Lopez. “It’s definitely a special relationship.”

Below, Mr Finney, whose Finney Fitness London studio is based in a private gym in King’s Cross, joins a panel of experts to guide would-be clients through the pitfalls and advantages in PT partnerships. Mr Dalton Wong is the founder and director of Twenty Two Training in west London, and has helped prepare Ms Jennifer Lawrence and Mr Nicholas Hoult for film roles. Ms Nathalie Schyllert’s clients have included Mr Hugh Grant and Mr David Gandy. She is now CEO of Bodyism, the exclusive Notting Hill fitness club. We spoke to them all to get the lowdown.

“The first thing you have to do is a meet-and-greet before you even start,” says Mr Wong. “What you’re hiring is someone’s expertise, but all trainers have that. It’s the relationship you build with your trainer that’s going to make the magic happen. So, I’ll interview my clients and they’ll interview me.”

Mr Wong encourages clients to ask questions and to take a look at a PT’s certificates. Good trainers will offer references or the chance to talk to existing clients. When it can cost up to £25,000 per year to have a trainer, it’s worth going the extra mile before you start. Mr Wong will also put new clients through a 90-minute preliminary assessment with one of his therapists.

“Trainers can be really quick to start diagnosing things and suggesting the same things without really knowing much about their client,” says Mr Finney, who also has initial meetings. “I might get vegans who want to bulk up, or steak eaters who want to lose weight. I met a new guy who has a massive weight problem after having cancer a year ago, but works insane hours in IT. I need to know all of this.”

It’s good to have high expectations, but, like a sculptor, you’ve got to work with the material in front of you. “Don’t tell me you want to look like Brad Pitt in Fight Club, because it’s not going to happen,” says Mr Wong. To a great extent, our physiology dictates the shape we can achieve, even with the most gruelling regimen.

There’s also age to contend with. Mr Wong says: “If someone says to me, ‘I want to lose 10lb, I exercise and did college sports,’ I say, ‘Yeah, bro, that’s 15 years ago. You’re not that guy any more.’ And I don’t want him to show me a picture in six months and say, ‘Hey, I was supposed to look like this.’”

When weight loss and shaping up is the goal (which is most of the time), Mr Wong presents new clients with two options. “I say, ‘I can help you achieve your goal in six months and there’s a chance it will come back, or we can do it in a year and there’s a good chance it will never come back.’ Some choose six months, but if you go for a year, it means we can do some really good things.”

Mr Finney and Mr Wong say their clients are rarely shy, but for first-timers in particular, it can be hard to know how much to submit to a trainer and how much to request. “People can be scared of asking for things,” says Ms Schyllert. “And I think in the industry trainers get into a rut where they just use the same programme for all members. But every single member is different and you should make sure the trainer is working for you.”

Like hairdressers and taxi drivers, personal trainers hear a lot of things. But in their case, they generally need to if they are going to help you properly. “When they’re physically exhausted, people tell me personal things and that’s when I capture it in the memory bank so when I’m trying to encourage them later I can use it,” says Mr Wong.

Clients who are reluctant to be totally honest about their lifestyles are only making life harder for themselves. “It doesn’t matter if you eat 10 Mars bars for breakfast, I just need to know so we can work with it,” says Mr Finney. Weigh-ins can be awkward, but Mr Finney says he is not there to judge. “I’m not looking at how much you weigh compared to me. I’m just looking for a marker,” he says.

“I’d never tell anyone off to make them feel bad,” says Ms Schyllert. “And the way a trainer responds to what a client says is also going to encourage them to be more honest in the future.” If your trainer makes you feel bad about yourself, look for another one.

For Mr Wong, honesty also means no shortcuts. Certain gyms are stacked with guys who are evidently not sticking purely to organic protein shakes, shall we say. “You can go to LA and get shots of human growth hormone, and testosterone will definitely help you bulk up, but if you eat well, train hard, rest and recover you can achieve great things without all that,” he says.

If you’ve got the time and money for a personal trainer, you should find one you trust and then follow their advice. For Mr Finney, the worst thing a client can arrive with is the conflicting advice of multiple experts in their lives. “I like people who’ve had a trainer before because they know how things work, but it can be a nightmare when someone’s already got a Pilates coach and three nutritionists and they start lecturing me,” he says.

“People will say things to me like, ‘My nutritionist says I’m not allowed eggs after midday,’ or ‘I can’t have milk in a hot drink because when it’s hot it turns to bad fat.’ It’s utter garbage and the challenge is that you have to suck it up and hope that bad advice will come to light and they’ll realise that I know what I’m talking about.”

It’s tempting to reach for that new pair of sneakers at this time of year and hit the streets to run off the bulge. But a good personal trainer should be able to suggest more effective ways to get fit than aimless bouts of cardio. “We know running makes you fit, but I don’t believe people should pay someone to watch them run,” says Mr Finney. “My business tends to be moderating weight and just looking a bit better, and you can run an hour a day and not do that. Also, the body can’t cope. I’ve got stockbrokers coming in on two hours’ sleep or guys who’ve been up all night in the studio and they don’t have the energy for it.”

Mr Finney is a resistance guy and has the arms to show it. “The most effective long-term strategy for busy people is weight training,” he says. While you’re running, you can go faster or run up hill, but then getting enough oxygen is the challenge. “With resistance, you can go harder, do different exercises and move faster between them. In two years, you’re burning way more calories, but only on three hours a week.” Moreover, with the guidance of a professional, you’re much less likely to develop an injury. Mr Finney also sets a handy goal for a lot of new clients: to work up to being able to achieve your own weight in 10 pull-ups, press-ups, squats and deadlifts. “When you can do that, your posture is better and you’re going to look better,” he says.