This is harder for me, my parents are quite well off financially and even when I was 32 they helped me with the downpayment on my first home. Hell, they still take my wife and I on family vacations at least once a year.
My wife and I both have good jobs, but it would be impossible to pay them back with any type of financial gesture, even a gift. They don't live an extravagent lifestyle but they have everything they want and need already.
Like @
Dadhawk said, I just try to be a good son, don't skip the family get togethers, call them every once and a while, take them out to dinner every now and again, ask for advice on things. Generally just try to keep their stress about me to an absolute minimum and include them in the happy times. Me being happy and successful makes them happy.