Simple... get hitched. All it takes is the same monotonous usage of their favourite expression, preferably perfect with the odd switch around and then a ring. The success of the star rating of the ring value depends on the target of your affections. Whore's will accept 1 star and upper class will insist on 5 star. Once they've taken the ring, you'll need to attach them to a house. So that means, a.) buying one and then b.) getting them to follow you to the house and then clicking on the sign out the front and selecting "Set as Marital Home".