I don't wash mine unless I absolutely have to. I find it takes the life out of them quicker.
I just keep them on the buffer and then turn the buffer over and turn it on. Then I take an air hose and shoot air staring from the center of the pad outward until the compound is gone. It should look relatively new once you're done. Especially on the foam pads. Just do it outside so you don't get your garage or car dusty from it. They also make a tool that has these spur looking things on it that works pretty decent to clean wool pads. I just prefer to use the air method.