Turn on your shower at its hottest temperature, close your shower curtain or door, and wait for a good 10 minutes, allowing the steam to loosen any surface soap scum or dirt.
Mix baking soda with warm water in a bucket. Coat a sponge with this solution and wipe down shower walls and tub, giving special attention to areas of caked-on soap scum. You can also use a synthetic scouring pad to better scrub these areas. Rinse the tub thoroughly.
Put on gloves and goggles, and dilute chlorine or hydrogen peroxide bleach in water in a separate cleaning bucket if soap scum persists. Apply either solution to the problem areas and let it sit only for a few seconds before rubbing or rinsing away from your porcelain.
Mix the tablespoon of trisodium phosphate with a gallon of hot water if particularly stubborn soap scum still remains. Apply it to a sponge or rag, and scrub soap scum away. You can also dilute ammonia in warm water for this same soap-scum-cutting affect. Afterward, rinse the tub thoroughly, and dry.