I use a product called "303 Aerospace Protectant." I buy 303 Protectant at Ace Hardware; but you can also get it online, too. Just a warning, it's expensive stuff (about $16 for a 16 oz. bottle); but it really does work. It's basically sunscreen for your sail: it protects the sails against UV rays. As an added benefit, for your case, it darkens the colors of your sail so that they look new again. Depending on how often you sail, I put the protectant on my sail about twice a year.
First, I give the sail a good washing with a gentle soap (not dish soap). Then after it dries, I apply the 303 Protectant, much like you would apply Armoral to your dashboard. It took me two 16 oz. bottles to do my Bravo sail (one bottle for each side of the sail). Then roll the sail up, your done. Note, I do not let the sail dry after applying the 303 Protectant. Instead I roll it up right away for storage so that the Protectant has a chance to really soak into the fibers. As expensive as that stuff is, I do not want to just let it evaporate off the sail before it has a chance to soak into the fabric.
Several months down the line if the sail begins to look dry, I just reapply, following the same procedure. This stuff is expensive; but so is a new sail.