I fill my tub with COOL (not cold) water 1/3 of the way, add 1/4 cup white vinegar, swish to mix, add the mat and soak each side of the mat 10 minutes. Some mats are oversized, so adjust it several times during the soaking process so all of it is wet. After the 20 minutes, take a SOFT brush (I bought a facial brush at my local dollar store) and gently rotate on the mat using a bit of Dawn detergent and this will loosen any remaining debris. Rinse with the same cool water, wiping as much of the water off of it as you can. Put down a towel and dry flat, flipping it every 1/2 hour or so until dry. Store FLAT.
I was shocked at how much lint came out in the water. Since most of the current mats are self-healing, that lint gets caught between the cuts. It's like having a whole new mat!