Ive had good luck with letting everything get fairly low (in the 40s around) besides health. i would clean the poops and stuff, but for the other meters, it seems to raise the health if you wait until he needs like 3 foods instead of just one.
also, instead of discipline, use the training function.... this doesnt make happyness go down.
and, if you need to leave for the day, set the clock to 12:01am and put him for a nap, he will sleep for 7 hours. (aka giga pause). 12:01 makes sure he doesnt forget his trick, and doesnt age it, but doesnt need you for a while.
need to leave again but stopping home in between, or just want him to sleep longer? if the clock is say at 4am, enter set mode again and put it back to 12:01. it will just keep sleeping and doesnt effect any meters! (used this alot in university). I seriously left one sleeping over 24 hours once and woke up to a normal giga. I did the pause trick, set it back a few times like i said, then i wanted to go right to bed so i then set it to 9:20pm (when it normally wants to sleep). it will then sleep the full night with you and, assuming you dont use the set back trick, he will wake with you the next day! it works!
thats the pattern ive used and ive gotten one to be.... i think it was over 40 years old at least!
ETA: I also do 1 bath and 1 vet daily. both raise the health. vet makes him unhappy so he will need a few games after. i dont bother with the vet if his health is already at like 99 (then the bath will make it 100).
