Perhaps a silly question but here goes ........ What is the best way to store boards so they don't twist or warp?

Background: I have slowly gathered a great quiver of boards, everyone a keeper for the long term. I have read about boards twisting in racks in the context of a board doing that and suddenly becoming either that magic board or loosing those special powers completely!

I currently house my boards on the rafters in the garage mostly in board bags. Good/bad or what is ideal for the long-term storage (in between surfs of course). Thanks