what is it you're not sure you're doing right? My general advice would be take it slow. only remove small amounts at a time. its easier to remove a little more bondo than to have to go back, re-bondo, then sand it again. trust me. also, for larger areas, i would suggest hand sanding, but thats just my personal preference. I bondo cars all the time, and once you get it roughed into shape, its easier to get it right by sanding by hand with a sanding block. if you really want to get the curves right, maybe even invest in a durablock and cut a smaller piece off it and use that. or maybe those sanding blocks with the grit on the block itself. Another tool ive seen that would probably be of some help is a 3m product i used at the shop. it was a spongy material like a thin layer of foam with sandpaper on one side. i'll try to find out more about it. its meant for wet sanding but i used it for dry sanding.
I don't know if this has been of any help, but maybe you'll find something you can use in there