Glass shower doors are very elegant. They are also a little sexy, and sometimes playfully scandalous, depending on the style you select and whether or not they are completely clear or have carefully placed etched sections. Whatever glass shower doors you choose, you should do your best to keep them clean. If they fog up, and the fog is not due to someone standing in the shower with the water on, you may have a more serious issue. Here is what causes the "fog" and how to fix it.
Soap is notorious for building up on shower surfaces, including glass doors. It will have a smooth, but thick and sticky, texture. Tub and shower cleaners can prevent the build-up and remove lighter layers of soap scum, but heavier layers need something stronger.
Since you cannot take a paint scraper to glass to remove the thicker layers of soap scum, you will need a cleaner that eats soap residue for breakfast. Lavishly spray this cleaner on the glass. Let it drip down and wait a couple of minutes. Then wipe clean. If your glass is not immediately clear and squeaky clean, take a second pass at it before resorting to something stronger.
Water that is heavy laden with natural lime causes an ugly build-up of thick lime scale on glass. You know it is lime because it is A) very white to light green, B) you can scrape it with a fingernail, and C) it refuses to come off with just glass cleaner. To fix this problem, buy a cleaner that targets lime scale, but is safe for use on glass.
Trapped Humid Air
This is a problem that high quality glass shower doors never experience; trapped humid air. The lesser quality glass shower doors have two panes of glass that are almost fused together to produce the correct heft and clarity. Unfortunately, this leaves some very slim spaces between the fused panes. After hundreds of hot showers, the trapped air becomes humid, condensates on the glass surfaces inside, and then it is a constant fog.
Not much can be done in this instance, except to buy a higher quality glass shower door and replace the lesser quality one. Clearly, you cannot extract the trapped humid air, nor can you break your shower door to release that air. Worst of all, if you tried to break the glass thinking you could fix the door, you would actually make the problem much, much worse (if you did not shatter the door altogether).