Who vs Whom

I can't say that I ever use the word whom, and I am pretty sure that I should be. So when should I be using it?
I went to Grammar Girl to teach me the trick!

• Who is used for the subject

Ex: Who went outside?

• Whom is used for the object

Ex: Whom did you ride home with?

If that is still confusing, ask yourself if he or him is the answer to the question. If it is him, you will use whom; likewise, if the answer is he, you will use who.

• Who went outside? He went outside.

• Whom did you ride home with? I rode home with him.

http://grammar.quickanddirtytips.com/who-versus-whom.aspx