Another nice read on this topic is this Scientific American article from awhile back:
The expert mind (Ross, August 2006)- "Experts are made, not born."
It's more enjoyable with the original typesetting and art work, but the text is
freely available here.
It makes the same point of working on problems at the right level:
"Ericsson argues that what matters is not experience per se but "effortful
study," which entails continually tackling challenges that lie just beyond
one's competence. That is why it is possible for enthusiasts to spend tens of
thousands of hours playing chess or golf or a musical instrument without ever
advancing beyond the amateur level and why a properly trained student can
overtake them in a relatively short time. It is interesting to note that time
spent playing chess, even in tournaments, appears to contribute less than such
study to a player's progress; the main training value of such games is to
point up weaknesses for future study.
Even the novice engages in effortful study at first, which is why beginners so
often improve rapidly in playing golf, say, or in driving a car. But having
reached an acceptable performance--for instance, keeping up with one's golf
buddies or passing a driver's exam--most people relax. Their performance then
becomes automatic and therefore impervious to further improvement. In
contrast, experts-in-training keep the lid of their mind's box open all the
time, so that they can inspect, criticize and augment its contents and thereby
approach the standard set by leaders in their fields."