Establish a legal structure similar to European systems. Clerics are employed to record all land grants or financial affairs. Charters are legally binding documents. If they are broken, punishment would be justified.
Threaten them with the authority of the Crown. If they turn against you, they and all their families will be killed.
Establish a land-holding relationship, wherein you grant money as a reward to the lords in exchange for their support and loyalty.
Which card will you play?