You can wipe the wire strings with a cloth or a strong paper towel. The guts and nylons I clean by slightly dampening a cloth or towel with water and a tiny drop of Ivory dishwashing liquid, wiping the strings on both sides, making sure nothing drips onto the harp, and then immediately wiping them with a dry cloth or towel. This removes any accumulated oils. This would only work on coated and nylon strings, and not very often. Wash your hands before playing! You can also put some talc on your palm.