This is a rough one that I am always looking up!!!
It's even a pain in the butt to have to try and explain so I am just going to let grammargirl.com take it away once again! Here is her 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. So that's the quick and dirty trick: if you can't remember that you use whom when you are referring to the object of the sentence, just remember that him equals whom."
...Yes! Finally a solution that makes sense! Thanks Grammar Girl!