If you need to sand a molding shape that is complex, a sanding block won't help. Not using a sanding block isn't too good of an option either since this can lead to uneven sanding. The best way to go is to build a custom sanding block to match the shape of the molding.

To do this, take a piece of scrap molding and cover it with wax paper. Then mix up sizable batch of auto body filler (such as Bondo) and apply it to the wax paper on the molding. Press it hard into the molding so that you get an exact match of the molding shape. When the filler dries, you can remove the wax paper, and you now have a sanding block that will match your molding exactly.