To increase just use them a lot.
No idea on exact numbers per heart; however, if you level your girls to 20 without using exp boosts in special training then you should have them at 4 hearts, or very close to getting 4 hearts. I have 3 lvl 20's right now and 2 of them have 4 hearts. The other one died a lot and only got 1 popularity point. Oh, and my girls are 3 and 4 star girls. Not sure if that makes a difference or not.