As latex balloons are biodegradable, it is normal that they oxidize and become cloudy over time as they degrade. Heat, sun and high humidity can accelerate the process, so you are doing the right thing by keeping your stuffed balloon away from sun and heat.
The producers of the Keepsake Balloon Stuffing machine (one of the major players in the area of balloon stuffing machines) recommend to coat the balloon with a silicon compound (for example "Stay Clear", "Balloon Shine" or "Balloon Shield"). If you spray the balloon with the silicon compound and polish it with a cotton glove, the balloon will shine more and stay clearer longer.
