The most important one that everyone forgets:
Have you been to the loo? There's nothing makes you want to go so much as knowing you can't.
If I've got rear seat passengers (PA28) I always mention which order they should leave the aircraft if they need to evacuate, i.e., I'll be the last one out, and that they should walk to the rear of the aircraft.
I have a modified one: "As pilot in command I shall be last to leave the aircraft. If I manage to get past you first then you become pilot in command."