One very simple way to maintain the appearance of your auto or RV windshield and to protect it from the elements and other irritating things is to use a cover. This can help to protect it from UV hazing and discoloration, from adverse weather, and from more homely things such as cats!
When your car is outdoors, naturally it is affected by the brightness of the sun and the temperature. So when it's parked outside, cover the windshield to lessen the effect. You can buy folding cardboard screens from the big box stores; these are erected on the dash or on the rear window platform. But, these are both inside the car. So while they protect the interior of the car from the temperature they don't do anything for the glass itself: the sun still radiates straight through and builds up heat between the windshield and the cardboard screen.
A better solution is an exterior cover. If you're vain or car-proud, you can buy these anywhere; they can be specific to your car or a general model. They can come with grommets so you can tie them down with bungee cords or strings; you may be able to get rear-window models as well. Prices range from $10 to $50. But if you're cheap you can get the same effect with simple vinyl sheeting: white for summer and black for winter. Cut it to size: width equal to the height of the windshield and about three feet longer than the width. Spread it over the windshield, slip the ends inside the car, and close the doors on them! You'll have to experiment to find out the best way to cover the rear window. This will keep out temperature, cat or other animal footprints, and such things as sap and caterpillar dung if you're parked under the trees in the spring.
If you customarily park outdoors in very sunny climates, the plastic may rot very quickly. You may want to go to a camping or outdoors supply store and check out space blankets. These are sturdy sheets of survival material: windproof, waterproof, non-heat-conducting, radar-reflecting things which will keep you alive if not comfortable in desperate situations. If the price is right, they will also do for windshield covers.
Finally, for those that live in the northern areas, where winters can be fierce, using an exterior windshield cover can make life easier and reduce the scraping wear and tear, both on you and the windshield! If freezing rain is forecast, or if you can expect the windshield to be frosted over in the morning, cover it up — as snugly as possible so as to make sure moisture doesn't get under the cover. Then in the morning just unpeel the cover, scrape the bits that need it, and you're off. Vinyl is better to use here than a regular cover since it will suffer from much tearing, being frozen to the windshield; much cheaper to replace.