The cliche the early bird gets the worm applies here. For some institutions, there are only a limited number of waivers they will approve. Provided you meet applicants meet their guidelines, waivers will be issued on a first-come, first-serve basis. In addition, some colleges and testing services require extra processing time for applications that are submitted with a request for a fee waiver. Missing these earlier deadlines or otherwise not allowing time for the proper reviewing of your request will result in denial.
The successful fee waiver application will illustrate that the fee is an undue hardship on your family. To accomplish this, it's not enough to simply report your family's income. The income must be given context. Do this by including information about the number of dependents in the household, whether anyone is unemployed or enrolled in school, and whether there have been any recent events which have negatively impacted your family's income.
In some instances, testing services or colleges may post the minimum requirements for a fee waiver. Ensuring that you meet these standards will not guarantee a fee waiver approval, but not meeting them will certainly guarantee a rejection. When guidelines are posted, they may include upper income limits, the submission of previous tax returns, and the signatures of parents or guardians. If no guidelines are offered, don't let this serve as a source of confusion or discouragement. Do your own research instead. Consult the U.S. Census for the average income of your state or the Federal Poverty Level and compare it to your financial situation.
Substantiate any claim made in your letter with proof whenever possible. When reporting income, include previous year's returns, unless instructed to do otherwise. If reporting the receipt of public assistance, include proof of this as well. Take nothing for granted. If you are claiming that well-known current events have placed an economic hardship on your family, include newspaper clippings as well. If there is a guidance counselor at your high school or financial aid representative at your college, see if you can get them to draft a letter corroborating your need for a waiver.