Chemistry also has some good ones -
to this day, I can remember my elderly, gray-haired chemistry teacher (17 years ago), prancing around the lab, singing:
H He
LiBeBCNOFNe
NaMgAlSiPSClAr
KCa
I appreciate that it doesn't have quite the same effect without the ludicrous musical accompaniment, but you'll just have to imagine that - it obviously worked though, 'cause I can still remember the elements order long after I've forgotten what they actually were.
The other chemistry one I know is a bit gruesome:
Little Johnny's dead and gone,
He'll be back no more.
For what he thought was H2O...
was H2SO4
And lastly, but probably most usefully are instructions for wiring a British plug:
You have a blue wire and a brown wire.
Blue has an 'L' in it and Brown has an 'R'
So, the blue wire goes to the left and the brown to the right
(and if they've given you an earth that only leaves one place for it to go)
