How do I clean and maintain my katana?

Do NOT put WD40 on it. You should try to get a micro-fiber cloth if you can. They sell these almost anywhere, they are often sold for cleaning HDTVs. If you can't get a microfiber cloth, a clean cotton cloth will do. Then you will need to get some mineral oil, make sure it says pure mineral oil on the bottle. Put a few drops on the cloth and wipe both sides. Do this about once a month, or whenever it looks like it needs it.