Start by boiling 3 cups (710 ml) of water in a kettle or in a pot over the stove. Bring the water to boiling temperature (212 degrees Fahrenheit, 100 degrees Celsius). If your pipes are PVC or plastic, 140 degrees Fahrenheit (60 degrees Celcius) is the hottest temperature water you can pour down the drain to prevent damage. Do a final flush of the drain with 3 cups (710 ml) of boiling hot water. This should remove any remaining grime or debris in the drain. You can flush the drain with hot water on a weekly basis to keep it clean.

Summary:
Boil the water. Flush the drain with hot water again.