This tip comes from Olfa.
CARING FOR YOUR OLFA CUTTING MAT
In order to clean and condition your green Olfa rotary mat, you can actually give it a bath. Put your mat in the bathtub and use 1/4 cup white vinegar to a gallon of tepid (not hot)
water, and a couple of drops (squirt! squirt!) of a mild dishwashing soap.
clean and condition your mat. Rinse the mat and let air dry or dry with a towel.
The mats love moisture. It helps keep them supple.
Be sure to store them flat and out of direct sunlight.