If you have hard water, then you probably have brown and yellow stains on your shower from the residue of sulfur and iron that is left behind when the water dries. While wiping down your walls will delay the formation of these streaks, it is nearly inevitable that you will have some sulfur stains to deal with periodically. Fortunately, there are several ways to remove these stains without a lot of fuss or hassle.
Pour some borax into a bowl. Leave the crystals dry for now. Wear rubber gloves to prevent the skin on your hands from drying out.
Dampen the scrubbing sponge with vinegar. The sulfur stains probably have some calcium in them as well, since hard water is high in calcium. The vinegar will break down the calcium and may help remove the entire stain.
Dip the damp sponge into the bowl of borax. The crystals will stick to the scrubber when you pick it up.
Scrub the sulfur stains. Use firm, circular motions. It is OK to press down hard on the stains because the plastic sponge will not scrape your tub. When you run out of borax or the sponge starts to dry, re-wet the sponge with vinegar and re-dip in the bowl of borax.
Rinse the shower down. Use plenty of water to remove all the vinegar and borax from the surface of the shower.
Dry the shower with a clean, dry rag. This will help delay formation of more sulfur stains in your shower.