First, if at all possible, store the vehicle indoors in a cool, dry environment. And make sure, whether it is indoors or outdoors, that the vehicle is impeccably clean. An indoor setting is the ultimate protection.One of the best ways of preventing the interior destruction of the engine is by making sure all of the fluids are fresh and full. For long periods purchase a fuel stabilizer, which prevents petrol breakdown (causing damage to the engine and fuel line) and allows you to keep the tank relatively full, which keeps out extra airspace. Airspace can lead to moisture and rust.
Take out the spark plugs and line the cylinder housings with oil. Again, this measure prevents corrosion. Change the oil and oil filter. Old oil has all sorts of by-products and acids that can cause damage.If the vehicle is left for extended periods take additional precautions. First, take the battery out and make sure to clean all of the connections. You might also want to jack the vehicle up onto axle stands and add about 10 pounds of pressure to each tire (if they are to remain on). This prevents flat spots. A good waxing will add extra durability to the paint finish.
Finally, take the wipers off so they do not become a permanent attachment to the windshield.The last thing you want is to get the vehicle back on the road, only to find that it's a home for rats, and mice. However, even though it is difficult to seal every crack a mouse could crawl through, it's easy to cover the noticeable holes (such as the tailpipe). If no one is taking the vehicle out for the occasional spin, drop moth balls around the outside for added protection.
(This from a colection of comments from various websites)