My copy of X-Men #1 went from a 9.4 to a 7.5 in two years because I didn't bag it right
I stored it loose in a long box with no backing board for two years after buying it at a shop in Dallas and now there's spine roll and a color break that cost me about $400 in value, so what do you all actually use for long term storage that won't wreck the corners?
That "long box with no backing board" part got me, because I did the exact same thing with a stack of Silver Surfer stuff in my parents' garage and forgot about it for like four years. Came back and the whole run had that wavy spine thing going on. Now I just toss everything in mylar with a board, costs more but I'm done learning lessons the hard way.