The old mnemonic my Dad can still recite from however many gazillion years ago (hahaha) he learned it:

My (Mercury)
Very (Venus)
Earnest (Earth)
Mother (Mars)
Just (Jupiter)
Served (Saturn)
Us (Uranus)
Nine (Neptune)
Pies (Pluto)

The new mnemonic he made up for the Pluto-less planets:
Miserably
Vain
Earthlings
Must
Just
Shut
Up
Now