Do unwarp boards with wetness and/or heat.  Wood, cardboard or other panels expands on its wet side (or heated side) curling downward.  Simply wet or heat the opposite side for awhile, and the board will straighten.  A board curled from laying on the ground will straighten simply by reversing it (while on the ground) until the warp disappears.

from The Do’s and Don’t of Yesterday by Eric Sloane.  A Treasury of Early American Folk Wisdom.