Is it who or whom?

On Erika's blog, Vicky gave an answer of whether to use who or whom (something I can never figure out):

Hey Erika I decided I'd answer the who/whom question for you. A good way to know which one works is to substitute who/whom for he/him or she/her in the sentence. If it's either he or she then it's who; if it's either him or her then it's whom. "Joyce is the girl who got the job." (She got the job). "She" works when subsituted so it's who since you wouldn't say "her got the job". "Mrs. Smith asked a psychologist whom she met in Florida." (She met "him" in FL). Also, use who when it's the subject of the sentence, and whom when it's the object. "Who is that guy driving?"--"who" is the subject; "The girls, two of whom are always late, were given detention"--"whom" is the object here. So that's it.

I figured it was definately worth posting - that way I can come back here when I'm having trouble remembering! Thanks Vicky!