What Do I Do if I'm Paying Too Much In Child Support?

I am a 25 year old father of two. I pay child support for one child and the other lives with me and his mother. I currently pay almost 400 a month in child support plus an additional amount in owed back support. before deductions on my check I make about 480, after taxes and child support, I bring home around 260. Recently i was denied for a modification of support amount. is there anything I can do???