Sending they to ride (and have some patience) have worked for me (in version 8.2). Per wiki
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That is 5$ daily and an investment of 1.700$ per girl in the cheap barn and you get in return 40% of happiness, 15% of discipline and +3 influence. Just the +3 influence per day at only 5$ cost it´s dirty cheap, and you can expect that each girl gain 12 happiness, 4-5 of discipline and +2 Dex per month. Not bad for a 1700$ investment.
In the expensive one the daily cost it´s 25 per day and the investment per girl 5.100$. But you get +6 influence daily (not so good, but still good) and you gain 25% discipline and 60% happiness. That´s +18 happiness per month and 7-8 discipline points per day. It´s not as efficient as the cheap one, but it´s more effective.
I usually put my 8 girls (vesrsion 8.2) in the cheap one as a SOP.
There are an increment in chances (no idea if big or small) if your girl it´s soulmate or better. Other ways rely on fear/low willpower, but there are not compatible with my "Prince Charming nice guy" way of playing, and I´m not familiar with them.