Summer months can be incredibly hot and sticky, so loose, lightweight and breathable clothing is essential. While linen and cotton are good options, cotton in particular does not dry well in Japan's humid climate. Spring and autumn can be crisp to cold, so you might want to pack thermal layers in addition to your coat and boots. Outside of Hokkaido and the mountains, the winter temperatures in Japan are manageable as long as you have regular cold weather clothes like a good coat, pair of gloves, a winter hat and a scarf.
The style of dress common for women in Japan may be a little more covered up than you are used to, particularly outside the major cities. For all travellers, packing comfortable trousers is essential as you may find yourself sitting on the floor, cross-legged, more  often than you would at home.
Read more about what to pack for Japan