I was fumbling with the usage of Who V/s Whom, found an interesting and useful read.
If you can’t get who and whom straight, try this trick: rephrase the sentence to get rid of who or whom. If you find you’ve replaced who/whom with he, she, or they, who is correct. If you find you’ve replaced who/whom with him, her, or them, then whom is correct.
For example, suppose you have the question Who/whom is the most famous A-list star eating lunch at Elaine’s? Rephrase it: She is the most famous A-list star eating lunch at Elaine’s. Since you’ve used she, you know who is correct in the original question: Who is the most famous A-list star at Elaine’s?
Another example: Vaughn wondered who/whom he would kiss next. Rephrase: Vaughn would kiss her next. Since the rephrased sentence uses her, you know whom is correct: Vaughn wondered whom he would kiss next.
Source : http://www.sparknotes.com/writing/style/topic_187.html