There are some companies that make products to slow UV degradation. Nikwax makes one, though I cannot personally speak to it's effectiveness. Apart from that, I think all you can really do is try to keep it out of the sun, whether by removing the bag from your bike when not riding or using a more easily replaceable rain cover to bear the brunt of solar rays.

http://www.nikwax-usa.com/en-us/prod...-1&fabricid=-1