In our game at the end of each session, the players usually thank the DM by saying "Thanks for running." I usually thank them for thanking me, or say "Thanks for playing."
My better players write me an email the very next day (sometimes timestamped late in the evening after the session has ended!) specifically thanking me for hosting, the hard work, and making it fun. In addition, when they can they specifically spell out their favorite parts and ask about hints I've dropped during the sessions.
By questioning his every ruling, complaining the encounters are too hard, the treasure is too little, building the most broken character build possible, and avoiding his plot hooks like the plague.