Avoid adding anything coloured to the storage area (i.e., use only white tissue paper, white or clear bins) to avoid colour transfer. Also store accessories separately for the same reason.
If it's hot and humid, try to ensure there is some air flow. I would avoid plastic bags altogether, and try to leave the bins not tightly sealed if possible.
Do prepare yourself to see some changes when you take out your collection next. Even in the best conditions ponies will change over time.
Good luck!
Elf