you just need to have great care of your pet, and treat it great. Dont let it be too heavy and take a good care and make sure it will be happy,.. your best care i guess <_<
If you got a Young Mametchi, you are on an odd generation (1, 3, 5 etc), and on the next generation you will not be able to get Mametchi. You will be able to get Mimitchi because it will be an even generation (2, 4, 6 etc) and then on the generation after that you can get Mametchi.
For your next generation, take the best care you can and remember to turn the lights out when it goes to sleep!