Ensure he never abuses Momo's power.

"So... I have another wish," he said slowly.

"Of course, whatever you want, if it is in my power to grant, I shall oblige!" Momo replied enthusiastically. She felt quite happy that she had pleased her Master by improving his mother's quality of life, and wanted nothing more than to keep pleasing him.

"I don't want to ever **** you or your power," Izuku said. "I don't want to ever do anything to make you or anyone else miserable for my own gratification. Your power contains temptation, and that scares me. I want to make sure that I never end up becoming the kind of person that would use others who don't want anything to do with your magic. Any use of your power should only be to help others, or for mutual enjoyment. Can you do this?"

"...I can," Momo smiled.

"Then I wish for that," Izuku nodded, bracing himself, prepared for whatever changes Momo needed to make to his psyche to ensure it.

Momo's eyes once again glowed, and she raised her hand to snap her fingers, before pausing. She then smiled, the glow fading as she lowered her hand. "Done, Master."

"...but... You didn't do anything," Izuku replied, blinking in confusion.

"Exactly, Master. You asked for something you already had. An incorruptible heart," Momo answered with a smile as she watched Izuku process the implications. She smiled all the more broadly as she saw him relax, with a bit of relief, and disbelief at the revelation that he didn't need a wish for what he had asked for. She realized that as far as Masters went, she had hit the jackpot. While her genie nature ensured she would have been happy to serve regardless, she most certainly preferred a kind Master. A legitimately good person to own her. Now she just needed to get him a little more comfortable with the fact that he owned her and could do as he pleased with her, and she would enjoy it just as much as he did. That would take a little time, to get him to be a little more assertive, but she had nothing but patience. This was his show, and he was well within his rights to take it at his own pace.