The misuse of who and whom is common. Ironically, people often confuse the two right when they are trying to be correct!
Who is properly the subject of a sentence or clause.
Who is going to the store?
John is the boy who is going to the store.
Whom is always the object of a sentence or clause, never the subject.
Whom did you ask?
Whom would you recommend we hire?
Is this the party to whom I am speaking? (Lily Tomlin)
Louise couldn't remember with whom she danced (or whom she danced with).
In today's English, most people consider acceptable such expressions as Who did you ask?, especially in spoken language and informal writing. In formal writing, however, you should generally use whom for the object of a sentence.
Consider the difference between the following:
The concert tickets go to whoever is the 10th caller.
The concert tickets go to whomever you wish.
In the first sentence, whoever is the subject of the clause "whoever is the 10th caller."
In the second sentence, whomever is the object of the clause "whomever you wish." You is the subject.