Treat her/him perfectly. Give disipline when needed, send it to bed at the correct time, clean its poo as soon as possible, and never let its hearts reach below 2.
still take perfect care of it! Put it to bed half an hour early,train it all the way,never let it's hearts go below 1 and watch it to make sure it barely ever poos on the floor!
Once my my teenage tamagotchi had it's hunger hearts got right down to zero, but it still evolved into a Mimitchi. That was the only time I got a Mimitchi. I can never take good enough care of it.