Anyone who's collected Magic for a while knows the unfortunate tendency for foils — especially older foils and the heavier promotional foils — to curl up towards the foil side (presumably as the cardstock expands from moisture). Unfortunately, this not only makes it harder to use the foils in decks (they become clearly marked — not a serious problem but still an annoyance for formats like Commander, and a more substantial problem for someone hoping to put e.g. some old foil Rancors into a Standard deck), but it presumably also decreases their value notably.
Unfortunately, just sandwiching the foils in between regular cards doesn't work very well — there isn't generally enough pressure from the regular cards to keep the foils flat; worse, this scheme doesn't really allow for storing a number of foils together (where they'll all curl together). Does anyone have any good suggestions for how to straighten out curled foil cards, or to keep foils from curling in the first place?