I don't know if it's the best way, but it's the way that works for me:
As soon as i'm done with a pad I let it soak in my wash bucket in clean soapy water using a little bit of car wash soap. Once i'm all done, I'll take the bucket inside and agitate the pad by hand using warm water and dawn dish soap. If I can't get all the product out with that method then I'll saturate the pad with dawn power dissolver and let it sit in a solution of warm water and oxyclean for about 10-15 minutes and then rinse and agitate again. Once clean, I either slap it on my rotary and spin it for a few seconds or I'll just set them face up on a clean bath towel and let them air dry. I've been doing it like this for a couple years now and have had no issues with mold.