This is probably a no-brainer for most piano teachers, but sometimes my brain works more slowly than usual. While explaining dynamics to a student this week, I simply correlated it to our speaking tones.
p=whisper
mp=talk softly
mf=normal talking
f=loud talking
ff=shouting!
It worked like a marvel, he played the whole piece with very good contrast. I may even try to make a poster for my studio to help beginners learn the differences in dynamic expression. :)