Installation of a water softening system can prevent most future problems. If that isn't an option, keep areas that get stained as dry as possible. Keep a squeegee and a spray bottle with lemon juice in the shower. Squeegee after every shower. Spray the shower walls and head with lemon juice and wash with a damp sponge once a week. Dry basins and faucets after use.
Once you clean off those nasty hard water deposits, it's best to clean them well and often. Otherwise, you will have to exert a lot of energy -- or resort to harsh chemicals -- to get them clean again!