The Buddha's instructions on gratitude to parents

A person of integrity is grateful & acknowledges the help given to him. This gratitude, this acknowledgment is second nature among fine people.

...there are two people who are not easy to repay. Which two? Your mother & father. Even if you were to carry your mother on one shoulder & your father on the other shoulder for 100 years, and were to look after them by anointing, massaging, bathing, & rubbing their limbs, and they were to defecate & urinate right there [on your shoulders], you would not in that way pay or repay your parents. If you were to establish your mother & father in absolute sovereignty over this great earth, abounding in the seven treasures, you would not in that way pay or repay your parents. Why is that? Mother & father do much for their children. They care for them, they nourish them, they introduce them to this world. But anyone who rouses his unbelieving mother & father, settles & establishes them in conviction; rouses his unvirtuous mother & father, settles & establishes them in virtue; rouses his stingy mother & father, settles & establishes them in generosity; rouses his foolish mother & father, settles & establishes them in discernment: To this extent one pays & repays one's mother & father.

Kataññu Suttas

http://www.accesstoinsight.org/tipitaka/an/an02/an02.031.than.html