Originally Posted by Ludens
However: to prevent is better than to cure. Low loyalty can be prevented by marrying princesses to them, improving your influence or keeping them close to your king. Succesful treason trials are also supposed to work (pick a low loyalty peasant or some such thing, as Ldvs suggested), but the few times I tried them I didn't found them very impresssive and if things go wrong you are in even more trouble. So I went off and attacked my neighbours instead. That worked pretty well.