You can keep hard water and soap scum in check by using a towel or sponge that,is soaked in lemon juice. The acidic lemon juice breaks down the build up of soap scum and hard water stains. If you have a lot of build up, you can use a commercial product designed for removal of hard water stains, since these are more aggressive. You could mix up your own home remedy by making a paste from cream of tarter and hydrogen peroxide. Spread this paste on stains and build up, allow to dry, buff, and rinse.