Products designed for plastic will prolong the life of your windows by replacing the oils that keep the plastic flexible. The single biggest thing you can do to keep your sails clean is to dry them . If you have to furl your sails wet, make it a point to shake them out when the rain stops and let them dry. Unfurl you genoa at the dock if there's little breeze, pull the main part way up, and let them air. Don't strangle the main when you put the stops on.
Tight stops cause unnecessary chafe on the sail as well as trapping moisture.
Make sure your mainsail cover fits loosely so there is air movement under it. It's also important to wash out salt and gritty dirt that can cause abrasion to the fibers. You can do this on a nice light air day with the sails on the boat. Drop the genoa on the fordeck, scrub the head with your deck brush and a mild soap solution, rinse, hoist that section to get it off the deck, and move down to the next section. Scrub, rinse and hoist until it's all the way up, then let it dry.