How and why you clean your shower mirror depends chiefly on what kind of shower mirror that you have. You can group shower mirrors, for the purposes of cleaning them, into two broad categories: (1) the coating-type mirrors and (2) non-coating-type mirrors.
Since — at this website — I recommend the non-coating-type mirrors, I’ll talk about those first.
Cleaning Non-Coating-Type Mirrors
These are mirrors that do not use a coating to achieve the fog-free effect. They may have a reservoir that you fill with warm shower water. Or perhaps they work by running warm shower water over them for a few minutes. But, the important thing is that they do not have a coating.
With this type of mirror, probably every four or five days, you will notice that the surface of the mirror will have “water spots” on it. These are mineral residues left over from the water droplets evaporating off of the surface of the mirror. The problem with the water spots is that they make the surface of the mirror less smooth, and, therefore, water droplets will not “sheet off” the mirror as easily. In other words, water droplets will tend to cling to the surface of the mirror if it has water spots.
You will always have water droplets on the mirror because it’s in a shower, but we need to reduce this as much as possible. Cleaning the mirror weekly can help a little with this.
To clean water spots off of the surface of the mirror, you simply need to wash the surface of the mirror with regular bath soap and a face cloth, and then rinse the surface of the mirror with water. I typically do this while I’m in the shower, usually once a week.
Cleaning Coating-Type Mirrors
Coating-type mirrors use a special coating to prevent fog formation. These coatings degrade over time. Low-quality coatings may degrade within a few days while high-quality coatings may degrade within a few months. The cheaper coatings more or less tend to wash off the mirror in a few days, which allows the mirror to begin fogging up again. The better coatings will often continue to prevent fogging but degrade in such a way as to cause visibility problems not related to fogging (basically the coating becomes somewhat bumpy due to coming in contact with steam and droplets from the shower day after day).
Whenever the coating either allows the mirror to fog up again or causes other visibility issues, the coating must be washed off the mirror. You can usually do this with a face cloth and bath soap. After scrubbing the mirror’s surface with soap, simply rinse it off with water. The higher-quality coatings may be somewhat oily and may require a few drops of dish soap instead of bath soap. (Or, using regular bath soap, you may need to wash it two or three times.)
Once you have washed off the original coating, you’ll need to retreat the surface of the mirror with defogging fluid. Or, you can actually just smear a little soapy water over the surface of the mirror each shower right before shaving or whatever (be sure to rinse the soap off after using the mirror). Another option is to rub shaving cream over the surface of the mirror and let the shaving cream dry. Then buff the shaving cream off with a dry face cloth. Shaving cream is itself an effective defogging fluid and should keep your mirror fog free for two or three showers.
I myself recommend that you get a reservoir type mirror or the type that you run hot water over instead. Check out my article Two Good Shower Mirrors for Shaving for more information about which mirrors I recommend.