Advice. We're bombarded with it every day - from magazines and adverts to well-meaning friends and colleagues. But only very rarely do we find advice that is life-changing.
In recent years, three pieces of advice stand out as having a particularly positive and lasting impact on my life.
1. To heal your body, sort your head out first
In 2003 I was a management consultant working long hours on complicated and stressful projects involving stock exchanges. As if the high-pressure job wasn't enough of a challenge, I had also suffered with chronic back problems for years. I was in near-constant pain, had stopped most physical activity and some days couldn't even get out of bed.
I saw the best doctors, chiropractors and orthopaedic surgeons, but their diagnoses were depressing. I was told I had a herniated disk and misaligned spine, caused by a hereditary spinal disorder which would continue to get worse. I felt angry and frightened.
The advice which changed everything came from my wife Kate, who gave me a book called Back Sense. She was re-training to be an osteopath and had recognised that my back problems might be related to my stress at work.
When I scanned the book's cover my first reaction was that it sounded like "new-age nonsense". As an engineer by background, I was convinced my back problems were structural; there was absolutely no way the pain was "all in my head". But it turned out that, although I was right about the pain (which was definitely not imagined), I was wrong about the causes - as were the medical 'experts' too.
The book explained how most back pain actually comes from muscle tension and that the underlying cause of this is often stress and anxiety. I was deeply unhappy at work, but because I wasn't acknowledging this, my mental tension and pain were appearing as physical symptoms instead.
Thanks to the book's advice I made some lifestyle changes to reduce stress and took up mindfulness meditation. I was stunned to find that within a matter of weeks I was almost pain-free and could begin to run and play sport again - having been unable to even walk just a month before!
Over a decade on, I feel fitter than ever and my back now acts as an early-warning "radar" which alerts me if I'm starting to become stressed.
Above all, this whole experience taught me that our mental health has a far bigger impact on our physical health than most of us - including the mainstream medical profession - have yet realised.
2. For big life choices, listen to your heart more than your head
Having finally recognised that it was making me miserable, I left my consulting job to pursue an MBA course, with the intention to move to a new industry.
The MBA included a unique opportunity to meet some very successful business leaders, who all sounded extremely impressive. But when we actually met them I was shocked to find they were self-obsessed and uninspiring! In achieving their "success", they had all lost something that money couldn't buy - like their health, marriages, friends, hobbies or values.
But one leader stood out from the others. He was warm and confident but also humble and self-aware. His career path had been diverse and unpredictable and included time working for charitable causes as well as running successful companies.
One of my friends asked him: "How did you plan such a varied and successful career?" His answer was one of my favourite pieces of advice ever:
"When a change or opportunity comes up I just ask myself - which of these things fills me with most energy and enthusiasm? Then I choose to do that thing, whatever it is"
His experience was that when we follow our instincts, we tend to make better life decisions than when we over-analyse or stick to the conventional path others expect us to. And if we're passionate about what we're doing, we also tend to do it better and enjoy it more.
Since that moment, whenever I've faced a major life choice I've tried to listen to my feelings. As a result I turned down the conventional corporate job offers after my MBA and chose to spend the next five years at the Carbon Trust, working on a cause that was much closer to my heart - climate change. And I never looked back.
3. Spend more time with people you love, doing things you care about
In 2010 I found myself struggling again. Although my job was much more fulfilling, my life remained out of balance. I was still working long hours; I was desperate to be seen as 'successful' by others - and I was often putting work commitments ahead of my family.
To help improve my work-life balance, I took the opportunity to work with a very talented coach called Aiden, who led me on a journey of self-discovery that had a huge impact.
One day Aidan asked me to imagine that I was nearing the end of my life. I had to picture what that would be like and how I'd feel looking back over all the highs and lows; the achievements and sorrows. Then he asked me a question which was devastatingly simple, but something I'd never really thought about before:
"What advice would your future self give you about what really matters now?"
So my third piece of life-changing advice actually came - somewhat surprisingly - from myself! Or rather from my "future self". And it was a powerful wake-up call:
"Spend more time with people you love, doing things you care about".
It seems obvious. But I was spending too much time obsessing about what I wanted to achieve and what others thought of me. And as a result I was getting increasingly consumed by my work and I wasn't there enough for my family.
Although I was passionate about my job, I also realised that it wasn't the best use of my strengths. What I really wanted to do was to encourage a greater societal focus on wellbeing and the things that really matter for a happy life.
So I quit my job to set up and run the Action for Happiness movement, which has been by far the most rewarding project I've ever worked on. Although many friends thought it was a strange move, it is an absolute privilege to do a job where I feel I can make a difference on issues I really care about; and to work with inspiring, like-minded people.
But above all, I also made a personal commitment to put family first. It's still a struggle to get the balance right. But over the last four years I've avoided working at weekends - and I've been there for countless school runs, choir shows, sports days and day-to-day family activities. And I wouldn't change it for the world.
So what advice has changed your life?
And what would your "future self" recommend that you do differently?
Follow Dr Mark Williamson on Twitter: www.twitter.com/actionhappiness