Gregs cabinet is a good idea... playfields are held in with slots and with the doors closed it eliminates dampness and temperature variations both of which will warp a PF in no time... once gone it can be a real bugger to get out.
I was always taught that timber should be stored standing up not lying flat and always out of any sunlight. Flat or standing it doesnt really matter so long as you eliminate temp variations between the top side and bottom... this is how timber curls.
My PF's are stored standing on an edge in a room that never gets hot and cold etc. I still though turn them around every couple of months as extra insurance.
