Try rolling them up tightly (like a rug) and leave them for several days. Unroll and reroll in a different direction. This may take several attempts at rolling in different directions.

Regards,

Terry

This is probably stupid advise, but what about wetting them and putting them on a flat board to dry, possibly weighted down at the edges....or do you have a liner sowed onto it??

Frans

Crease them the opposite way for awhile, it works to some degree.