Kissing while fucking boosts love till, I think, 500 or so. Other than that, orgasms, gifts and dates. Sleep with them, the morning sex carries over the orgasm count from the night prior, it gets quite silly when they are coming every other action.
Something I've found which appears to work even at high levels of Love (have to have pretty high levels of Obedience) is to tell them they aren't allowed to masturbate. They will often call you and ask for permission. So far, every time I mercifully allow them to relieve themselves, I have gotten +11 Love.
Now I WILL say that this command doesn't work QUITE the way it should, as many of the girls can still be caught rubbing one out when you go to their room. In addition, if you don't 'catch' them, their Lust just drops from 90+ to 10+ overnight (indicating that they schlicked off without permission). Perhaps they should add a spanking dialog for when they disobey you (I mean, you can already smack dat ass, this would just be an extension of said smacking).
I'm still running my play through, since I haven't maxed everyone yet. So far, one thing I haven't gotten yet which I think is unusual is I haven't gotten calls from the girls getting together on their own. That usually happens by this point.
Also, I did finally trigger the girlfriend prompt (on Kitty), which quickly triggered my side dish prompt (Storm). The first two girls who become your 'girlfriend' are the ones that give the approval for the others entering your harem. Still trying to be a bit of a jerk, when Kitty asked if I was getting serious with Storm, I even tried denying it or turning her down (after saving, just to see what happens). Funny enough, if you just say 'yes', you only get +5 red. But if you deny it, if they have a close enough relationship, she will actually ASK you to bring in the other girl, and you get a +10 red instead.
I know that new mod is supposed to show the girls relationships, but if you don't have it, then you can often see their compatibility by having them make out with each other. If they refuse, very low compatibility. If all they do is a lot of kissing, medium compatibility. If they get downright freaky with each other, then good compatibility.