For light cleaning, mix one part water with three parts mild soap (like Woolite) in a spray bottle. Spritz the mat, and wipe it dry with a soft cloth or towel.
If your mat is heavily soiled, fill the bathtub with cool water and add a drop of mild soap. Scrub with a soft cloth, soak for a half-hour, and scrub again. Rinse thoroughly. Place the mat on a dry towel, and roll them both up, jelly roll–like (so the mat is inside), to squeeze out excess water. Unroll mat, and hang up to dry.