As the sail is exposed to UV it slowly starts to lose it's tensile strength. Generally the fading of color in the sail is a good indication of how much UV the sail has been exposed to.
When you're not flying, try to position the glider in a shade. Many gliders get a large chunk of the UV exposure while still on the ground. If you can't find a shadow, one trick is to try to rotate the glider so it generates the least amount of shadow. When the shadow is the smallest, the sail receives the least amount of sunrays.
Many pilots reported good success with this product which is safe to use on Dacron.