If you have self cleaning mats, such as olfa, they can be soaked in a bath tub, and a soft brush can be used.
I used distilled water and a drop of dawn dish detergent. I lay my large mat on a towel on a folding table I use. I have a dish brush (kind you fill with soap) I use that. When scrubbed lightly. I go over it with clear distilled water and then allow it to dry. Then go over it with a quilting mat smoother (not sure of the correct name). The board is like new.