Sorry everyone
I forgot to mention that it's not that useful to get kings high command ratings, as it has been seen that 9 star king can get a heir with 4 stars.
Using a general is way better, as the general never loses his stars, unless he gets some very bad vice, but even then he can still reach 9*, as there is virtually infinitely time for him to fight.
Just try to get your generals to be preferably heavy cavalry, or at least heavy infantry, and if they're weak units, keep them out of battle at any costs, for I have bad experience...
6* peasant general was standing over there, enemy cavalry made a charge against his unit, and guess who dies in the frontline to the charge