I was attracted to Teach for America for many reasons--a chance to make an immediate difference, the challenge, chance to move across the country, free masters degree, alumni network, grad school applications--and recruited by Teach For America to be a recruiter myself. Thus, I had already worked as a Campus Campaign Coordinator at the University of Southern California for almost 4 months before applying.
Essentially, make sure that you do research on education inequity, the progress that Teach For America has made, and that you're joining the movement for the right reasons. The application process is becoming more competitive, and it boils down to qualified people are turned away because they don't have the knowledge about or drive to fix education.
Draft responses to address the following issues: a difficulty you have overcome, a success story despite obstacles, your understanding of public education. Avoid generalizations and statements about education that you cannot quantify or speak for personally.
Also, make sure you do at least one test-run of your lesson plan with someone who has taught. They will offer excellent advice of how to clarify your instruction.