It looked like these options from the wiki are probably the most effective way but they're all kind of random so IMO you want them all active at the same time to increase your chances:How do you train discipline? Wiki says put her into riding school. Bought best equipment for Amelia, left her in riding school for 100 days and she hasn't gain a single Discipline point.
Only when she developed workaholic she started gaining it.
- Explaining chores: By successfully explaining a slave's chores, you have a 15% chance of increasing her Discipline - if it's 49 or lower.
- Devotional gain: Maintaining a Willpower below 9, Happiness above 60, and Affection that is Loving or higher, provides a 1% daily increase chance - if the slave's Discipline is 59 or lower.
- Fear: Slaves with 1+ Fear has a 20% chance to increase their Discipline - it it's 79 or lower.
Fear sounds like it has the best chance of improving it while just maintaining willpower below 9 is probably the least work once you get it there (especially if you have advocates at negative willpower). That's my plan anyway. Juno is kind of a pain to break initially but I figure she'll save me time in the long run once I get her broken. Explaining chores also has the advantage of reducing willpower in addition to the chance at increasing discipline although it kind of tanks her happiness so you probably can't do it every day.