How do you make people pay their member fees?

I am an officer for organization and there a lot of people who have not paid their fees. It is a big organization so the fee is quite high and we told them in the beginning of the year. We were suppose to have set dates for payments but things got complicated with the sponsors. I know this year was rough but we really need the money now so we could get things done. How do I make the members pay their fees, and we can't just kick them off the organization. We also do not want any commotion with their parents, we have always had financial problems and this year I want to fix it but I do not know how to...