#195--This works well. I've used it for the shower drain at my studio:
1/4 cup baking soda
1/2 cup vinegar
2 quarts of boiling water
Pour baking soda down the drain. Follow that with vinegar and then cover the drain for 30mins. Last, slowly pour the boiling water into the drain, and you're done. No caustic chemicals to damage your skin, your eyes, your lungs, or OUR water supply.