In reality there IS a way of getting free on-board credits (but your not going to like it). You would be part of a group sailing that is offering free on-board credits as a perk to the group. Sometimes Princess will offer a credit in thier individual bookings but in reality, YOU are paying for it. Sometimes a free credit for a missed port. Do they give credits just beacause your a good guy or special occasion? No.
I have more time in ship elevators then you do in cruise days. Its just not done.