Step 1: Pour 1/2 cup baking soda down the drain.
Step 2: Pour 1/2 cup vinegar down the drain and immediately cover (to create pressure) and let sit for 15 minutes.
Step 3: Pour a full kettle of boiling water down the drain.
A number of commercial drain cleaning products use compressed air instead of chemicals to open clogs. Enzymatic drain cleaners may be a safer alternative to chemical drain cleaners, and they're easier on the environment. They use bacteria or enzymes that naturally feed on organic waste. Find these at grocery or hardware stores.
For best results, use a drain cleaner monthly to prevent clogs and keep drains smelling fresh. For stubborn clogs, use a mechanical snake.