Relentless self-improvement.
In other words, learn and train, and then test yourself in the real world, in a situation where it's possible to lose.
It seems to me that doesn't fall within the definition of munchkining provided. Assuming for the sake of argument that it's good advice, I think people refrain from following it more out of akrasia and to avoid the bad feeling of losing than because they don't see it; think it's impossible, unsporting, etc.
A munchkin is someone who follows the letter of the rules of a game while breaking their spirit, someone who wins by exploiting possibilities that others don't see, reject as impossible or unsporting, or just don't believe can possibly work.
If you have done something that everyone around you thought would not work, something that people around you didn't do after they saw it work, please share your experiences. If you tried something and failed or have ideas you want to hear critique of, likewise please share those with us.