Well, you can place it in an airtight container with a dehumidifying agent such as silca gel,limerocks, or sodium chloride. Just place a piece of wax paper or parchment paper between your sugar and the agent do not place them directly on it. I am guessing you are using regular granulated sugar as your mixture? You would really be better off with tartaric acid if its available in your area. Other than adding a dehumidifying agent thats about all you can do, that or use it quick! Or just place it in an airtight container and hope for the best I wish there was more I could tell you but spun sugar is very fragile and will not last exposed to humid conditions. There is one more thing place it in zip lock bags that might hold it for a while.