If you are not on fresh water, minimally the sail should be hosed off throoughly and wiped dry with a sponge to remove salt. Once it has dried it should be carefully inspected for holes, chafe, and worn thread areas. Cringles, eyes, headboards and other hardware should be inspected for wear and attachment. Any problem areas should be repaired. I prefer to roll my sails rather then fold them but they should be carefully rolled or folded and put away in a cool space and protected from light.
I don''t like the idea of using detergents, although many people do use mild detergents, and I don''t like the idea of putting sails in a washing machine.