1.  Put gas stabilizer in the tank
2.  You can add air to the tires and/or buy some of the tire "ramps or pads"
(can't think of the proper term) that redistribute the weight of the car so
as not to flat spot the tires.
3.  And...of course...use one of the Porsche or other microprocessor based
battery maintainers
4.  Note:  With any of the newer (996, 986) cars, the remote receiver in the
car will shut down if not used for a week or so.  You have to use the key to
start the car and the receiver will turn on and operate normally.

Be careful of mice.  Long term storage seems to attract the little critters.
I suggest you raise the car on cincer blocks where you put some tin sheet
between the block and the car so they can't climb in.  Crack the windows,
put some moth balls inside, don't, repeat don't put a cover over it.  A
cover traps moisture which causes corrosion and rust. Make sure the garage
is not air tight and hopefully there is a good flow of air around the car.
If you decide to drive it, drive it until it is at operating tempature for a
while.  Starting the engine and letting it idle and then shuting it down is
a bad practice.  Also, disconnect the battery and remove it to keep it from
freezing and cracking otherwise keep a charge on it.

I live at a ski resort and do very little in the winter.  The car sits
outside under a Noah cover and I keep a battery tender on it.  I try and
move it whenever possible, to keep the tires from flat spotting, and take it
for a spin when the roads and weather allow.  I also put fuel stabilizer in
the tank.  I have the oil changed in the spring and have my mechanic go over
the car.  Runs like a bat-outta-hell every year with no problems.