Properly Storing a Car Through Winter
Clean the interior and exterior of the car thoroughly before storing it. Stopping dirt and dust from getting under the car cover in the first place is the best way to keep it clean, and apply a fresh coat of wax just beforehand, to ensure that it is nice and shiny when you pull out in the spring. If you decide to steam clean the interior, be sure to give the car plenty of time to dry out, or you could get mold growth inside. Some desiccant purchased from a dollar store can help keep moisture levels to a minimum.
Any untreated metal exposed on the car is liable to rusting, so purchase a can of rubberized undercoating to protect it. This coating is flammable, so be sure to keep it away from any parts that get hot. If your storage location is potentially open to small animals, stuff a sock in the exhaust pipe to stop them attempting to nest there (since it can be difficult to guarantee that no mice will ever enter your garage, this is probably a good idea for everyone). Cheap drier sheets in the interior can dissuade them from setting up shop if they somehow find a way in. Mothballs work as well, but the smell makes them a less desirable alternative.
Tires will always lose pressure slowly overtime, and the loss is only accelerated by the kind of temperature change they will experience in the wintertime. Inflate your tires before storing the car, but be careful not to go above the maximum air pressure the tire is rated for. You should double check the pressure on all four tires before removing your car in the spring. If one of your tires has a leak, it will completely deflate and the car will end up resting on its rim; be sure to replace any faulty tires before storage. For older cards or those who just want to go the extra mile, jack stands can take all the pressure off the tires. This can be a tricky procedure, so ask a professional or highly experienced friend to help you.
Most ethanol-based fuels are only rated for three months before corrosion in the fuel lines and the engine can set in. A fuel stabilizer can prevent problems down the road. Be sure to check the antifreeze levels and replace the oil and oil filter if you can, taking the car for one last ride so the fluids circulate properly. Batteries can crack in the cold, so consider removing it to warmer storage.
A quality car cover can make a sizable difference. If your car will be outside, make sure the cover is waterproof and secure enough that high winds don’t blow it away. It is important that your cover is breathable and keeps moisture at bay. You can use a good quality tarp's. Some claim that you should start a stored car every so often; the truth is, if you’ve done a proper job storing it, it should not be necessary. If you do decide to give it a whirl now and then, be sure to remember to replace everything you removed and unblock anything you blocked (like that exhaust pipe!)