You can place most foam pads in the washing machine if you have a "hand wash/delicate" setting and then throw them in the dryer on "low" or "fluff dry" setting.
I personally would not recommend using either the washer or the dryer because the dryer, even onlow settings, tends to cause the glue holding the pad to the velcro to wear out and then release the pad.
I clean mine as I use them, or at least I try. If Iam buffing a car and my pad needs to be cleaned I'll just hose it down, wring it out and put it on my machine to spin dry.
When I get home I do exactly as "rtbrick" does except I usually use Simple Green instead of dish soap.
Anthony