Have her imitate Lucy.

"So Lucy... Any ideas on how to play with her besides just making her a slave outright?" Natsu whispered in Lucy's ear.

Lucy beamed, glad her Master was going for other girls, and was utilizing her for it. Jealousy didn't even register with her in this mode, her top priority was Natsu's happiness, that was what made her happiest. To be allowed to aid him in getting other attractive girls, even if it was just a suggestion, filled her with joy.

"How about... Make her want to imitate every action you have me do?" she whispered back. "And after she has done it once, she'll do it again any time you want..."

"Great idea," Natsu smiled.

"Oi, what are you two whispering about?" Cana blinked, still covering her tits.

"Sorry, we're just talking about a mission we're considering taking," Lucy lied easily as Natsu took out the Note. "In the meantime, we just wanted to hang out..."

"Cana Alberona will want to imitate any action towards Natsu Dragneel he has Lucy Heartfilia as her slave alter ego do, regardless of how she felt previously about it. Once she has already done and action for him once, she will be eager to do it again for him upon request from then on." Perfect, a nice gradual enslavement for her.

"Oh... Hey, could I get in on the mission?" Cana asked.

"Nah, it's easy and doesn't pay much, dividing the pay further when I got upcoming rent isn't really desirable," Lucy said apologetically.

"Oh, that's fair," Cana nodded, not taking it personally. She understood the pain of crunch time for rent all too well.