"Playing a Grand Piano Risks Ever-Decreasing Enthusiasm" is the key to no less than eight separate mnemonics, that will help you in such diverse areas of life as playing musical instruments, geology, and astronomy.
Pregnant Camels Ordinarily Sit Down Carefully: Perhaps Their Joints Creak (geological eras)
All Cows Eat Grass (spaces on bass clef)
Green Buses Drive Fast Always (lines on bass clef)
Please excuse my dear Aunt Sally (order of mathematical operations)
Richard of York Gave Battle In Vain (colours of rainbow)
Every Good Boy Deserves Favour (lines on treble clef)
Dumb Kids Prefer Candy Over Fancy Green Salad (taxonomic categories in biology)
Eddy Ate Dynamite Good Bye Eddy (guitar strings)
I seek no reward for my invention, other perhaps than to be serenaded yearly on the guitar and piano, while a choir of geologists, taxonomists, mathematicians and astronomers dilate in four-part harmony on the mystical powers of the goddess Mnemosyne. Is that too much to ask?