Keep the tires from ground contact...use inert plastic blocks such as the orange Tri-Max Lynx Leveler Blocks that keep the tires dry and off the moist ground. Spray them with 303 Aerospace Spray to keep them from ozone damage and uv damage.
Ozone and UV damage are the big tire killers, and as said above...proper air pressure.
If possible, a short drive every couple of months will re-distribute the natural waxes that are added to the tire compounds by the manufacturers to keep the tires from drying out and cracking.
My posts shouldn't be taken for factual data. They are purely fictional, for entertainment purposes and should not be constituted as actually related to scientific, technical, engineering, legal, spiritual or practical advice. Amen.