If you are tired of listening to your generals to see if they may desert in the next round, try this. Leave yourself some awards, save the game, select DONE in the Domestic Mode. Your chancellor will tell you who left your troops. Now simply remember these names and restart the game by pressing A + B + C + Start.
Start the saved game, promote the 'disloyal' characters and they won't desert. If you have no awards left, you can donate a Faith Coin to them. This is also useful if you need to save your awards (and soldiers) for your main fighting characters.

Mercenaries and thiefs normally get unhappy if they were used in combat and get no awards afterwards. To prevent them from desertion, deploy them to far-away castles for potato planting, but don't let them fight. They will not make trouble any more.