I had also read that you can soak the mat in the tub, however this is only for Olfa mats. Even on their website they recommend soaking them once a month as they love water. But beware, not all manufacturers recommend this.
I have a Fiskars mat and I called them to see if they recommend the same thing. They told me "NO!"
So please check with the manufacturer of your mat before soaking it.
I also heard that the scrunchies work great, but haven't tried it.