I learned quite a bit about tyres from a post on another forum, where someone had a problem and went to Michelin for advice. Michelin sent a very, very detailed answer which was extremely informative. The following is paraphrasing from Michelin's response.
Tyres that are not used often, ie when a car is off the road, are adversely affected by two things; UV light and oils in the rubber compound not moving.
So, if a tyre is sitting still for prolonged periods the oils in the rubber 'settle' and make the tyre stiff which causes cracking and degradation.
There is also a third factor, which is linked to the oils. This is a tyre sitting on a cold, hard surface in one place for prolonged periods. This causes the oils to harden more at the point of contact with the ground.
Pumping the pressures up to higher than optimal operating is of no benefit, apparently, and can cause more damage due to the oil issue above.
Their advice was to keep tyres indoors (say a garage), keep them as warm as is possible, move the tyres as much as possible and insulate them from cold surfaces.
I keep my car(s) in a garage all winter with the tyres sitting on four pieces of old carpet to insulate them a bit from the cold concrete. I jack the car up every few weeks and rotate the tyres by 90 degrees or so each time. It is a faff but tyres are expensive, so who wants to throw them away if they can easily avoid it?
