"Whom" - does anyone have a simple non-technical guide to usage?

No English teachers, please. I don't want to hear that it's an object of a verb or a preceding proposition - that doesn't do diddly squat when I need to know when to use it. Anyone have a nice memory trick for "whom" (along the lines of "the executive administrator of a school is your pal - so he's the principal, not the principle")?