Mnemonic devices suck.
They don't work because they depend on rhymes but there's lots of rhymes that often fit in the place of the 'correct' one.
For example, I never remember the date of Columbus's arrival to the new world, because ANY fucking date that ends in a two works!
For all I know, it goes "Columbus sailed the ocean blue in the year 1982"
Another example is I never know how many days there are in february because I fucked up the mnemonic for it in my head with this abombination of it:
"30 days has September, April, June, and November - All the rest have 31, except February, which has NONE."
So in conclusion - Mnemonic devices can suck my ass.