Lower the screen, vacuum or dry-brush the dust out of the weave, apply a mild soap solution with a cloth or sponge, work it in small circles with a soft brush, and rinse thoroughly. Leave it down until it is completely dry, then raise it. Twice a year is enough for most homes; near salt spray, more often. Go easy with bleach: the mesh handles it, but the thread holding the panel together does not, and the seams give out first. If you do not know the thread, use a peroxide-based cleaner instead.