I just store mine away from sunlight and extreme temps, in a well ventilated area, and don't worry.  I don't wear muck boots because my foot is so wide and they are so expensive.  I've found a neoprene type boot at Ace Hardware that fits like a dream, and cost about $50.  Shoe Goo will patch any hole.  I suppose you could loosely pack them with dry newspaper in the off season.