Hello!
A simple "thank you" note goes a long way.
Definitely! And maybe an email or a phone call every other year to let your instructor(s) know what you are doing flying-wise. Nothing beats an "I was just promoted to captain of an A340"-message to your ex-instrutor!
I know an instructor who was given a very expensive watch by one of his students. He is so embarrased that he never wears it, which makes this the most useless present ever given.