Did Euler have a trick for discovering things? Some sort of general method he could apply to mathematical objects he came across to see if they yielded any new truths? Did he just ask the right questions at the right time? Was he lucky? Did he have a very advanced and well-developed intuition?
How would you train a kid to become the next Euler. Just imagine you were given a kid who was really good at math, and it was down to you to give him all the training needed so he could embark on a career like Euler's (and you are guaranteed he would work as hard as Euler). For example, if I worked all day exploring maths I probably wouldn't discover a quarter of what Euler discovered, and even then I'm being HIGHLY generous to myself.
How did he do it?
Robin Wilson wrote about Euler (see comment):
Leonhard Euler was the most prolific mathematician of all time. He wrote more than 500 books and papers during his lifetime — about 800 pages per year — with an incredible 400 further publications appearing posthumously. His collected works and correspondence are still not completely published: they already fill over seventy large volumes, comprising tens of thousands of pages.
Euler worked in an astonishing variety of areas, ranging from the very pure — the theory of numbers, the geometry of a circle and musical harmony — via such areas as infinite series, logarithms, the calculus and mechanics, to the practical — optics, astronomy, the motion of the Moon, the sailing of ships, and much else besides. Indeed, Euler originated so many ideas that his successors have been kept busy trying to follow them up ever since; indeed, to Pierre-Simon Laplace, the French applied mathematician, is attributed the exhortation in the title of this article.