How can I effectively sand concave surfaces?

To effectively sand concave surfaces, use sandpaper with a flexible backing to conform to the curve. Start with coarse grit sandpaper to remove imperfections, then switch to finer grits for a smoother finish. Use a sanding block or sponge to evenly distribute pressure and avoid creating flat spots. Sand in a circular motion to follow the curve of the surface and regularly check your progress to ensure even sanding.