Ideally, a heated garage is a place where you would want to store your Spider. Not all have access to one. If you must store it outside do not put it on grass or dirt. Both attract moisture and we all know what that leads to. An excellent quality car cover should also be used. Wash and wax as well.
These little rascals will chew wiring, upholstery makes nests and leaves remnants everywhere. There are many opinions as to what works best. Anything from D-con to mothballs, peppermint oil, electronic devices, even dryer sheets. Do a little homework and pick the best for you and your Spider.
Most fuel is now ethanol based. There are some stations that offer non-ethanol but not in many areas. Ethanol is hygroscopic, it attracts and absorbs water. Having a vented system will create condensation which together with the ethanol fuel the water will settle in the bottom of the tank, Spiders having metal fuel tanks which can cause rust.
Be sure before you put your Spider away to have the tank fuel. This will make less of an area or space for condensation to form. Adding a fuel stabilizer will help. Be sure to run your Spider for a few minutes to allow the stabilizer to circulate thru the entire system.
If your Spider is only going to be stored in a heated garage for a brief period of time and the battery is in good condition disconnect the negative cable so it will not run down.
If you plan on storing it for a longer time or in an unheated garage and have access to electricity a trickle charger is a good option to keep the battery charged. If no electricity is available remove the battery.
It's also a good idea to change your oil before storing. Fresh oil will help to keep your engine lubricated during its winter nap. The extra bonus is when it's time for that spring drive you’ll be ready.