The very last line:

To improve problem solving, one should study solved problems, attack the problem while in different moods, and try explaining the problem to others

  As recapped in bold at the end by the author:

Problem solving can be thought of as search on a graph.

Insight is distinguished by a change in problem representation.

Insight can be facilitated by active seeking of new problem representations [ie drawwing, analogies]

Incubation [taking breaks during working] enhances problem solving ability.

A night’s sleep improves problem solving ability to a considerable degree.

Mind-wandering [ie low effort tasks] facilitates creativity.

[One] should study solved problems, attack the problem while in different moods, and try explaining the problem to others