David Wethey's Twitter account describes him as a "survivor" from the Mad Men era of advertising, but he’s much more than that. Having thought he would embark on a career in teaching (a car crash on his way to his final interview put an abrupt stop to that path), he switched to marketing. Now with more than 50 years of experience, he's seen the way firms advertise themselves change beyond recognition. In fact, he helped shape that evolution through his firm, Agency Assessments – itself a spur-of-the-moment idea that's turned into a 28-year business and yielded two influential books.

What inspired you to start doing things differently in the advertising world?

I hadn't written some defining business plan for Agency Assessments – it wasn't a grand scheme. A client [called] looking to completely rework his marketing organization, which wasn't fit for purpose. Then, coincidentally, another friend [called] and said they wanted to buy a PR agency. So I found myself working for two clients, not as a direct agency but as a kind of consultant intermediary.

This role developed into Agency Assessments International (AAI) – we’re a consultancy that helps our clients to hire advertising and other communications agencies and work effectively with them. Nobody had done that before, but to me, it made more sense. By consulting directly for clients, we could be completely objective in our recommendations – rather than pushing one particular agency – making us an ally for clients.

Tell us about a lesson you learned the hard way.

I've always been exceptionally enthusiastic about the "new" – new clients, new projects … new attempts to push my golfing handicap down to single figures. Agency Assessments is, by nature, a project-based business, so I'm constantly looking for new opportunities. Because of my focus on what’s coming next, I was never as good as I should’ve been at maintaining relationships with my existing clients. Ironically, this has led to missing referrals for new contact opportunities.

I’ve also learned that you need to work on becoming a bit of a thought leader. It was my history teacher who showed me that it’s possible to do that – he pointed out that just because somebody had written a book, it didn’t make their words true. We can all bring our ideas to the table. Now I’ve promoted the business through articles, workshops and seminars, and realized that my opinions and views can be of the same value as those of the "experts."

How important is it to occasionally step back and reassess?

Completely. I hadn't realized it until I came to write the first of my books. If you've sat in what used to be smoke-filled rooms – they've now been sanitized, but there's still a lot of hot air in them – deciding which agency you're going with, as well as the whole science and psychology of how people and companies make decisions is fascinating.

The key thing in my book, Decide: Better Ways of Making Decisions (2013), was that I stepped back and interviewed 25 great decision makers and learned an enormous amount. We're taught to be logical, but gut feeling is also incredibly important in everything we do.

You also wrote Mote: The Super Meeting (2015). What distinguishes an average meeting from a super one?

So many times I used to wonder who was supposed to be leading meetings, what the agendas were meant to be and why so-and-so had been invited – or left out. At the end, people would just shuffle off to their next meeting without any real conclusion. But since setting up Agency Assessments, it’s been the other way around. For every meeting I've organized, I've known what the objective was, who should be there and how long it should take. I had a clear expectation of what the outcome should be, and would share it with my client.

What are your tips for staying at the top?

Never stop learning. One of the depressing things about experienced people is how often their experience is just an aggregation of doing the same thing the same way, over and over again. A great deal of young people think that people in their 70s are running on empty – some kind of a throwback to the old days. But people who have stayed at the top, who have managed to succeed, are inevitably open to new ideas.

David is based in our Pall Mall office in London. His books are both available to buy online. Follow him on Twitter @davidwethey for updates on advertising, decision making and a dog named Alfie.