Useful Mnemonics

For me it's not even close:

"30 days hath September, April June and November"

I invoke this handy aide d'memoire at least once a week when referring to dates past, present, and future, and have ever since I was a kid.

Others in my commonly referred to list:  

I V X L C D M — Just stop whatever you're doing and memorize this. Roman numerals made simple: 

1 5 10 50 100 500 1000 . 

Now you'll be able to translate inscriptions on buildings etc. wherever in the world you find yourself. It can be fun at parties to show and tell people their ages in Roman numerals.

"Red sky at night, sailors delight; red sky at morning, sailors take warning." I've followed up on this over the years and it's quite accurate.

ROY G. BIV — I think it was in elementary school that a teacher told us to memorize this name so we could quickly recall the colors of the rainbow in order, short to long wavelengths.

HOMES — the Great Lakes made easy: Huron, Ontario, Michigan, Erie, Superior