I usually have a bucket of warm soapy water standing by and as soon as I'm done polishing, I throw them in there to soak until I can can get to them. Then just hand wash with warm water and some low sudsing detergent, rinse throroughly and air dry.
For the record, I do not use Festool polishing pads, but can't imagine that my cleaning process would be detrimental to the Festool pads.