Yep. Keep a Oniontchi or Young Mimitchi's hearts filled all the time and never let it go below two, keep it at good weight, clean up its mess and try to avoid toothaches.
To get Oniontchi take good care of your Marutchi.
To get Young Mimitchi take good care of your Kinakomotchi.
I've gotten many Mametchis/Mimitchis in my past. It easy take top care of them and if your in school pause them. I know pauseing sounds bad but after you unpause just set the time back and let it run til you have to go to sleep, thats how I make up for missed time from pausing them at work. *points to Admin* It does work.