Stop while it is still good

Games with grown-ups often end badly because they go on too long: someone gets tired, someone cries, and that is the last thing everyone remembers.

End on a high. When the game is going well, give a warning: "Two more turns and then it is bath time." Then really stop, and say one thing you liked: "I loved how your dragon guarded the bridge."

A game that ends while everyone wants more is the one they will ask for again tomorrow.