You can also cast-in-place sanding blocks with some packing tape on your surface, some polyester filler (Bondo) to take the surface shape and a wood backer to hold on to...
I'm a big fan of the 3M yellow foam blocks and the Stickit paper too - the blocks are hard enough yet have some give. Not cheap but a great system.
You can also glue acrylic sheet to softer foam to make a flexible-yet-hard block for curved surfaces. You want to sand (fair) with something that takes the shape of the surface you want to achieve - but no more - so you hit the high spots but don't over-sand the low ones.