into a gallon of hot water (not boiling, to avoid cracking the pipe). Pour this "slurry" slowly down the drain; the salt lowers the freezing point of the ice, much like it does on sidewalks. Step 3: Repeat.

are best for preventing these pipes from freezing again next winter?

Once the drain pipe is unfrozen, you are not done. You must act quickly to prevent flooding and future problems.