I got a fee waiver based on my parents income as well as other issues that I detailed specifically in my letter - parents filed bankruptcy some years ago, father is unemployed, mom only makes around 1600/month, medical bills. ANY unusual/extreme/unfortunate circumstance - mention it.
I also got a fee waiver for pretty much the exact same situation, except both parents got laid off and had to file for bankruptcy. I originally got denied because the tax forms they requested from my parents were from when my dad was still making aroun $100K. I was making around $22,000/year at the time. I was very detailed in my letter including what my current bills where (i.e. rent in a city where the cost of living is ridiculous, high student loan payments, insurance, etc...). I listed the calculations of how much I was making per month after taxes minus all my bills and how with such little disposable income I would be unable to afford the application fees. I think I included documentation of my parents' current lack of income, bankruptcy court filings, as well as the lay-off letters for both my parents.