Sine = Opposite over Hypotenuse, Cosine = Adjacent over Hypotenuse, and Tangent = Opposite over Adjacent.

 Mnemonic:
SOH-CAH-TOA

 Mnemonic:
Some Old Horses Can Always Hear Their Owner Approaching

 Mnemonic:
Some Old Horse Came A-Hoppin' Through Our Alley

 Mnemonic:
Silly Old Henry Can't Add Hundreds, Tens Or Anything

 Mnemonic:
Some Old Hags Can't Always Hide Their Old Age

 Mnemonic:
Some Old Hippie Caught Another Hippie Tripping On Acid