This is a word that is destined to die, and I can't tell whether or not I'll miss it when it does.
It has already died a half-death, in that you never see it at the beginning of a question, even if it is the proper word to use. (You'll never hear "Whom were you talking to?", although you might occasionally hear "To whom were you talking?") Beyond that, though, I intend to use it when it should be used (and I remember to do so) elsewhere until the word completely fades from the language.
There is a simple rule that tells you when "whom" should replace who: Who is always a subject, and whom is always an object. More tellingly, if replacing the word with "him" (or "her") makes sense, then you should use "whom". If the proper replacement would be "he" (or "she"), then "who" is the way to go. Note that you may have to restructure the sentence as a statement for this to sound natural in some cases.
Another big tip-off is a nearby preposition. ("To whom do you owe allegiance?") Unless I'm sorely mistaken, the object of a prepositional phrase is always... well, an object. Thus "whom" instead of "who".
At least you don't have to worry about changing "wherever" to "wheremever"...
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