Drywalling?

I am completing an abandoned drywall project and the frame is not exactly flussh and all the boards are already hung, i am having difficulty giving the apperance of a flush level surface without sanding my self into the next millenium, i have already spent too long, i have been using those sponge sand block things that the old contractor lefte behind for the job, i have a hunch that htis is where i ama going wrong, please help me