When a Christmas cactus is wilted, the first thing to do is to cut back the branches by 1/4 of their length, to encourage new growth. Make sure to keep the soil moist and provide enough sunlight, but not too much as this can cause the plant to burn. A light fertilizer can also be applied to help the plant recover. Finally, provide a cool environment for the cactus, keeping the temperature between 60-70°F to encourage healthy growth.