In Thinking as a Science (1916), Henry Hazlitt wrote (my emphasis):
The secret of practice is to learn thoroughly one thing at a time.
As already stated, we act according to habit. The only way to break an old habit or to form a new one is to give our whole attention to the process. The new action will soon require less and less attention, until finally we shall do it automatically, without thought—in short, we shall have formed another habit. This accomplished we can turn to still others.
I agree and have been advocating this for years. People learn to do something correctly, once, and then think they've learned it and they're done. But that's just the first step. For skills you'll use often, you should practice until you can do it cheaply, easily and reliably. E.g. it's important to be able to type using almost zero conscious attention so that I can focus my attention on the ideas I'm writing. It's best to think in an objective – not biased – way pretty much automatically in general so that you can focus on considering a specific topic (like economics); people who need to use a bunch of mental focus to avoid bias are at a big disadvantage because they have less attention left for the actual topic (and what often happens is, at some point, they focus their attention on the topic and then their habitual bias starts happening).