This is a method used for many years in taking care of a flattened and less-cushioned futon because of the long-term use. The best season to practice Yotsuyu-boshi is around a day of ox in midsummer which the sun is the strongest.
Leave the futon out in the balcony all night long to expose it to the night dew. After letting the futon soak up the night dew, expose it to the sunlight during the day. And repeat this process, so it is fine to leave the futon out for a couple days. This method would help the cotton batting regain softness and elasticity. This method is available only for 100% pure cotton batting. This method isn’t suite for a synthetic batting.