Wow there are soo many different jokes to ease the tension during gear-up and in the plane, a couple of my favorites are as follows and please feel free to use them...
1) tell the student 'oh one more thing, when we leave the plane and during the freefall please keep calling out "ORANGE HANDLE, ORANGE HANDLE" it helps me remember.
2) Casually introduce yourself to another Tandem Instructor gearing up thier student (of course so the student can hear) "you must be the new guy, and is this your first day?" when they reply yes nervously, say "welcome aboard".
3) last but not least when you land and are walking in, mention to the student so that thier friends and family can hear, "hey, thanks for taking over when I froze up-up there, shh lets keep that between us ok ...hehehe that usually gets them all laughing!
Be safe, have fun