The basic rule for when to use whom over who is the following:
    Use the he/him method to decide whether to use who or whom.
        he/she = who
        him/her = whom

Here's some examples to help clarify how to use them.

Eg.1 Who/Whom is this book about
It is about him/her. Therefore whom is correct.

Eg.2 Who/Whom is going on the trip
He/She is going on this trip. Therefore who is correct.

Eg.3 Who/Whom left the milk out?
He/She left the milk out. Therefore who is correct.

You should now have a much clearer understanding of when to use whom over who. Although in reality whom offers no extra value or clarity than simply using who. You can go about the rest of your life and never use whom, nobody would be the wiser. Although if you do use whom it automatically makes whatever you said classy and distinguished. Take for example:

     To whom should I deliver this rotten banana to?
Classy isn't it.
 

So there you have it. Use who when referring to he/she and use whom when referring to him/her, or when you feel like classing up your sentence. 

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