To prevent soap scum build-up, squeegee the shower walls and doors after every use, or wipe them with a towel. Apply a light coating of lemon oil to tile walls or frosted shower doors. For a porcelain tub, apply light coat of boat and car wax to the sides—and only the sides—of the tub.
To deal with existing build-up, use a degreasing agent and scrub thoroughly. Use an alkaline soap scum remover from a janitorial supply store, or dissolve a handful of dishwasher detergent in a bucket of warm water. Then, cover the affected area completely, and let the solution soak in for a minimum of 15 minutes. Scrub thoroughly with a stiff brush or sponge, and rinse.