• 1/2 cup baking soda
• 1 cup (or more) vinegar
• Boiling water
• Salt

1. Pour 1/2 cup of baking soda down the sink drain, then and add at least one cup of vinegar. Put the cover on the drain and wait a few minutes. Remove the drain cover, then rinse through using a mixture of boiling water and salt.

I found this on the web somewhere? Can't remember where?