Cure For Curled Souvenir Sheets

I just got my hands on a couple of hundred Russian souvenir sheets from the 60s, 70s and 80s which have curled with age, some to a point where they keep slipping out of their photo-corners. A local chap suggested I put them for a month or so in one of those photo albums with plastic over a tacky back but I ruined some SSs that way once before. Any ideas on how to flatten them out other than keeping them pressed inside a heavy book for a generation or two? Thanks in advance.