I typically hand wash mine in cold water with a scoop of washing powder in the tub. I had one I tried in the machine once, and it was torn to bits (the rubber) so I just hand-wash them now.
Don't hang them to dry in the sun if you can avoid it, I've also found the rubber can stiffen with regular doses of the sun on my mats - I'm not sure if its cheap rubber or what, but I hang them in the shade now.