I've never had to do that before, but I'm not making nearly what I should. One problem is one of the offices I work for is a charity, and they really don't make a lot of money.
"I need a raise"
Simple as that. Believe me, I was a manager of a major retail drug store and I didn't like to hear sad stories, how poor you are, or how much you need it. I respected a employee that just came right out and asked for a raise. When you are in management you are taught it's all about the bottom dollar, and if the company is having a hard time, then they will NOT care about your situation because if they can't give a raise then they just can't and a sob story will not change that. Now this applies to big companies, NOT mom and pop places of employment.
But yeah just tell them "I need a raise" But never add "or I will quit" never give an ultimatum unless you plan on following through, because if you say you will quit and they say sorry no raise, and you do not quit then you just lost all credibility and respect. Trust me.