Anyone have any advice on dealing with the CSA?

Hi all, im new to this site and also have recently been contacted by the csa and the case worker assigned was aggressive and very unhelpful. From reading the posts i can see why now. They have calculated what i need to pay by contacting the inland revenue for my p60 details, which from what i can see that the amount they are using does not exclude any pension contributions i have made. Does anyone have any advice they can offer when dealing with the csa.