Address your letter to the appropriate party. This small gesture shows that you've done your research and genuinely care about the status of your financial aid. Call the school you're attending or will be attending, or check out the school's website to find out the name of the head of the financial aid department.
Start the letter by explaining your gratitude for acceptance into the school and for any financial aid they've already offered you, if applicable. If the latter is true, go into detail concerning how the current standing offer of financial aid --- while appreciated --- still leaves too much of a financial burden on yourself or your family to attend college.
Make a case for your financial aid by using your or your parents' finances --- depending on who the school is factoring into your financial aid allowance --- to point to concrete statistics about how you simply can't afford your education. Include factors such as a mortgage, car payment and other necessary bills that prevent you from taking out another loan for your education.
Discuss your qualifications as a student that make you a prime candidate for financial aid. For instance, describe past accomplishments that prove your worth to the college, such as a high GPA, extracurricular activities and notable awards. Write graciously and end by thanking the financial aid head for her time. Reiterate the fact that you can't attend college unless you get the help you need from financial aid.