I flatten prints in "pockets" of glassine, which is inexpensive, and recommended to me by a museum conservator. I stack up the pockets, so there are two layers between all prints except the top one, then release paper, close the press, turn on, allow to get warm to the touch on the top outside, turn it off, and come back when cool. Very flat, although, as some have said, 3 months later when I take them out to frame, depending on humidity, they can re-curl.