In a perfect world, the wheels should be removed from the car and stored (see the next article). If possible, the suspension of the car should remain compressed to avoid the onset of rust on the shock absorber piston rods.

Should it be impossible for you to store the car without its tires, inflate them to 35 PSI (2.4 bar) or 40 PSI (2.75 bar), depending on the maximum pressure on the sidewall. Ideally, during the storage period, move the car for a few centimeters once a month or, if the tires are high performance tires, every two weeks to avoid the formation of flat spots.

If your tires are stored mounted on wheels, keep them inflated to a pressure of 15 PSI (1 bar).
Ideally, do not store them in an upright position, but stack them laying flat on their side. If this is impossible, leave them upright and rotate them about once a month to prevent the formation of a flat spot.
Protect them with plastic bags thick or dark enough to protect the tires from ambient light.
Avoid extreme temperatures and humidity. Above all, refrain from storing them outside, where they will likely be subjected to temperature swings and high humidity.
Do not leave them on a moist or greasy floor; isolate them with a wooden plank.
Keep them at least two meters away from electric motors and radiators. These devices generate ozone which oxidizes tire rubber and deteriorates the tires.

Originally posted by Mike Wazowski
What about gas stabilizer?