What are the best techniques for sanding and smoothing wood rough after applying polyurethane?

After applying polyurethane to wood, the best techniques for sanding and smoothing the rough surface include using fine-grit sandpaper in a circular motion, sanding with the grain of the wood, and finishing with a final light sanding to achieve a smooth finish. Additionally, using a sanding block or sanding sponge can help to evenly distribute pressure and avoid over-sanding in one spot.