Grammar Girl's Quick and Dirty Trick wrote:
Still too hard to remember? OK, here's the quick and dirty tip. Like "whom," the pronoun "him" ends with "m." When you're trying to decide whether to use "who" or "whom," ask yourself if the answer to the question would be "he" or "him."
That's the trick: if you can answer the question being asked with "him," then use "whom," and it's easy to remember because they both end with "m." For example, if you're trying to ask, "Who (or whom) do you love?" The answer would be "I love him." "Him" ends with an "m," so you know to use "whom."
But if you are trying to ask, "Who (or whom) stepped on Squiggly?" the answer would be "He stepped on Squiggly." There's no "m," so you know to use "who."
Linky I love Grammar Girl so much....