Appears in large friendly letters on the cover of the HitchhikersGuideToTheGalaxy, which is perhaps why it has sold so well.
Good advice in most situations.
ThereIsNoSituationSoBadThatYouCantMakeItWorseByPanicking.
Take an AmigaComputer, activate its narrator device, and execute this algorithm:
volume = 10
pitch = 440
speed = 2
for n from 1 to 100
volume += 1
pitch += 10
speed *= 0.99
speak (volume, pitch, speed, "Don't Panic!")
sleep (speed)
The effect sounds absolutely terrifying...
Also a catch phrase of 'Mr Jones the Butcher' from a British cult comedy TV series from the 1970's/80's called "Dad's Army" see - http://www.mgnet.karoo.net/dads%20army.htm Corporal Jones