Well, to begin with, this type of buildup is usually nothing more than mineral deposits. The minerals are in the water that flows through the faucets and can settle out on the fixtures, particularly in the shower where a lot of the water you use evaporates. When the water evaporates, it leaves these mineral deposits behind, and they can cause quite a buildup over time.
In areas where the water is very hard, this type of buildup problem will be greater, but it is actually a relatively easy problem to fix. The best way to get these mineral deposits off of your shower and faucet fixtures is to allow them to soak in vinegar overnight. After an appropriate period of soaking, the mineral deposits should come off easily with moderate scrubbing.
But no matter how many times you remove these deposits, they will always come back. They are simply a result of the makeup of your local water, and so unless you take greater action, you will have to repeat this process on a regular basis if you want to stay residue free.
However, there is another option. Rather than having to soak all of your fixtures in vinegar every so often, you may want to consider having a water softening system installed in your home. These systems are appropriate in areas where the water is very hard and they can go a long way towards eliminating buildup of sediments and minerals all over your home water system.