Basic dish soap and water works quite well. If the pot is unglazed (that is the standard brown flower pots) scrub with a scrub brush to remove the residue and let air dry. If you are concerned about bacteria, you can place a dry pot in the oven at 200 degrees to sterilize them. Leave about 20 minutes. DO NOT sterilize a glazed pot with this method. Use the standard bleach rinse to sterilize them.