How do you unclog drains without using caustic, over-the-counter products?

Pour 1/2-cup baking soda down the drain. Add 1/2-cup white vinegar and cover the drain if possible. Let sit for a few minutes, and then pour 6 or more cups of boiling water down the drain to flush it. The combination of baking soda and vinegar breaks down fatty acids in soap and glycerin, allowing the clog to wash down the drain.