With the first few it might be difficult/impossible, because you're short on staff and I don't think there's an option to avoid them having sex with customers until later in the game. But as your trainer skills improve and your roster fills up, it gets way easier to mange their virginity... That sounds so wrong.
Around the midgame-mark you will be able to buy (rare) items in the shop that increase their max stats. On that note, try and empty the shop before a payment day comes (keeping your payment in mind), as items get more expensive afterwards.
Another way to increase the max stats is new game+, so keep your favorite girls around until the end.