In order to discourage rust, corrosion, and sun damage, you want to choose an indoors location that is dark and dry, with a stable temperature; the ideal place is in a garage or indoor public storage facility. If you do have to store it outside, be sure to find a durable, weatherproof car cover to maintain the paint and limit exposure to the elements; choose wisely, since a defective cover can trap moisture and grit. Be sure to cover any large gaps where critters (particularly rats) can enter, such as the exhaust pipe or air intake.
Wash and wax the exterior of the car to clean and protect the paint; any leftover dirt or contaminants (such as water stains, bird droppings, mud, grease, or tar) will corrode and rust. Clean and vacuum the interior as well, since, again, dirt and debris can cause damage through mold and mildew if left alone for an extended period of time.
Make sure your tires are inflated to the recommended pressure to prevent flat spots. You can use plastic sheeting to prevent moisture from degrading the tires. You may consider taking off the wheels entirely and placing the car on jack stands at all four corners.
Top Off Fluids
You should have fresh fluids for your fuel systems. Oil and filter, anti-freeze, gas, power-steering fluid, tranny fluid, and brake fluids should all be chnaged right before storage. Old fluids, particularly oil and antifreeze, can become acidic acidic and eat away at the insides of your car, potentially causing mild corrosion. Full fluid reservoirs allow no room for condensation to occur. You should purchase a fuel stabilizer as well to prevent ethanol buildup and protect the engine from gum, varnish, and rust.
Keep the Battery Charged
Most newer vehicles are equipped with advanced computer systems that require constant power, and an unattended battery will eventually lose its charge. The low-tech solution is to disconnect the negative battery cable, which might cause you to lose your settinngs such as stereo presets and time. Otherwise, you can purchase a battery tender (a.k.a. trickle charger) that hooks up your car battery to a wall outlet, delivering just enough electrical power to prevent discharging.
Disengage the Brake
Do not use your parking brake when keeping your car in storage, since the brake pads might fuse with the rotors if in contact too long. Instead, you can use a chock (a.k.a. tire stopper) to keep your vehicle in place. Leave the gear in neutral for manual transmissions.