What are the keys to making an empire healthy?

Founded by Genghis Khan in 1206, the Mongol Empire was the largest in world history. How did Genghis Khan manage to create a vast empire that continued to expand after his death?
First, the empire was built on the Mongols's extraordinary military prowess. The Mongols relied on cavalry who used bows and arrows. Fighting on open plains, the Mongols closed with enemies only after wearing them down with arrows. The Mongols also learned how to lay siege to cities.
Second, the Mongols wisely chose not to immediately attack every city they encountered. Surrender was allowed, and the local rulers would then swear loyalty to the Khan. Cities that resisted were destroyed, and their inhabitants were killed or enslaved.
Third, the Mongols had an extensive spy network. Because of this, they were better informed than their enemies. Foreign governments were often suspicious of Mongol diplomats, thinking they were spies.
Fourth, the Mongols gradually learned how to administer their vast empire. In the beginning, they merely exploited conquered lands. For instance, Genghis Khan had to be persuaded not to turn northern China into pastureland. However, they later began to use the local machinery of government to manage territories. Religious toleration was granted, and that helped hold the empire together.
Finally, Genghis Khan deftly prepared his sons to rule the empire after his death in 1227. His sons and their successors even managed to further expand the empire. By 1241, Mongol armies were at the gates of Vienna.