Eat everything you take!
Although not a rule, from childhood, Japanese children learn that they should not have any left-over food as it is wasteful. Therefore, in order to behave like a local, try to take a small amount of food at a time. As it is an all-you-can-eat, you don't have to worry about not having enough food! The fun part is eating and walking around the restaurant to see what delicious food is offered. So remember to take a small amount of food so that you don't leave any food behind!