Watch the servants scurry; though they move quickly, they are a stealthy and observant lot.
Hold your tongue, or you may need to cut theirs out.
Set up well-paid spies among them, especially in the kitchen.
Keep them from windows, and out of proximity to the archer openings along the walls and stairs.
Limit their food.
Search their rooms frequently; their privacy is a myth, and nonexistent in your home.
Let them rest, that their service upon their return is better, and faster; beating them defeats the purpose, unless you enjoy it.
Pay them enough to provide for their families, but never enough to purchase their freedom.
Let brides be with their husbands on the night of their wedding; do not claim rights to her virginity, for a cuckold is a disloyal soldier, and a lifelong enemy. Remember that power is transitory, and fleet of foot.
Turn the females of the realm to your purpose. They will persuade the men through the art of love, or the arts of poison and assassination.
Keep the ratio of mercy to ruthlessness balanced toward ruthlessness, and they will be all the more grateful for the mercy. It may be possible to quell rebellions thus.
In the matter of public executions, read the crimes before the crowd, and use an experienced headsman. Keep them youthful, but train them to cut clean. The suffering of an improperly beheaded subject is unforgettable, and may induce fear, but it will be tainted with anger.
Show up at their superstitious festivals, and as they celebrate, scan for potential enemies. Those who attend alone or stand in the shadows should be rounded up quickly as possible, and questioned extensively, with violence if need be.
This is the way to keep peace within your walls, and the realm, with those who rely on your generosity for their survival, and indeed, their existence.