I forget when I ran across this, but it is helpful to me in remembering prepositions and what different combinations of preposition + case indicate.

(Thanks to John Schwandt and his site BiblicalGreek.org for the link.)

The site in question is called New Testament Greek: Prepositions. It is one of a number of lessons. After you go to the page, scroll down to section 2, "Examples" until you see the cartoon. Yes, it's a cartoon. But boy-howdy does it work!

Enjoy!