What are the best techniques for applying sand before polyurethane to achieve a smooth and durable finish?

To achieve a smooth and durable finish when applying sand before polyurethane, the best techniques include sanding with fine-grit sandpaper in between coats, ensuring the surface is clean and free of dust, and applying thin, even coats of polyurethane. Sanding in a consistent direction and allowing proper drying time between coats can also help achieve a professional finish.