"So he should never let his thoughts stray from military exercises, which he should pursue more vigorously in peace than in war. These exercises can be both physical and mental. As for the first, besides keeping his men well organized and trained, he should always be out hunting, so accustoming his body to hardships and also learning some practical geography..."
"As for intellectual training, the prince should read history, studying the actions of eminent men to see how they conducted themselves during war and to discover the reasons for their victories or their defeats, so that he can avoid the latter and imitate the former."
"...A wise prince should observe these rules; he should never take things easy in times of peace, but rather use the latter assiduously, in order to be able to reap the profit in times of adversity. Then, when his fortunes change, he will be found ready to resist adversity."
from 'The Prince'
by Niccolo Machiavelli