Here are some methods I use:
Shaving Cream
Before you shower, wipe some shaving cream onto your bathroom mirror.
It will keep it from fogging up so you don’t have to wait to get to
work with your toiletries or shaving after you get out of the shower.
For me this works really well. Here is how it works:
thenakedscientists.com
Dave - In some senses, it doesn’t actually stop it steaming up.
Steam is lots of little droplets of water. When the light hits it,
the light gets bent and so you get a very distorted image which, when
you move away from it, just looks like a kind of mist. What the
shaving foam does: there are lots of detergents in it and those
detergents reduce the surface tension of the film of water so it
doesn’t form lots of little droplets. It just forms a big flat sheet
that you can see through much better, so you can see through it even
though the water is still condensing.
Ben - So there’s literally the same amount of water there; it’s just
a change in the structure of the water. So why do the droplets make
it so hard to see things?
Dave - Because water has quite a high refractive index - when light
hits it, it bends. If it hits this curved surface of a droplet, each
one basically acts as a little tiny lens, light is bounced off in all
sorts of different directions and makes it looks essentially white.
This breaks up the image and makes it look misty.
1 quart spray bottle
2 tbsp. household ammonia
1/2 cup rubbing alcohol
3 drops dish soap
1). Pour 2 tbsp. of ammonia, 1/2 cup rubbing alcohol and three drops
dish soap into a 1 quart bottle.
2). Fill the bottle with tap water, tilting the bottle while filling to
minimize suds.
3). Screw on the spray mechanism tightly to avoid leakage. Your anti-fog
glass cleaner is now ready to use in the same manner as a store bought
glass cleaner.
For me this works up to 10 minutes and works on glass lenses for glasses, as well. This works better if you apply several coats and can also be used for cleaning the glass.
Creating a 50-50 solution of vinegar and water will also do the trick.
Instead of buying a mirror cleaner like Windex, which has a ton of
chemicals inside, you can use this all-natural cleaner instead.
Simply pour a cup of water and a cup of vinegar into an old spray
bottle, add a drop or two of dishwashing soap and spray. Wipe the
mirror with a towel and the glass should stay steam-proof for a few
days.
Vinegar does have an odor, but it should go away within an hour. You
can also add lemon juice for a fresh smelling bathroom.
Also using blow dryers are suppose to be:
if you forgot or don't feel like applying anything to your mirror, you
can use a blow dryer to quickly eliminate the water from your mirrors.
Is there a technique to make a shower mirror fog-free?:
This Q&A has a lot of methods, that can be bought.
Some methods for making these products more effective are:
Turn a fan on while you shower.
Take your mirror down while you shower and put a towel over it. Wait 5 minutes after a shower, and then put it back up. Or apply the towel while it stands up.
Also, using thicker products like shaving cream and makes them stay longer. Soap with the water in the air can wash off.
Putting warm objects on the mirror works, as well.
Additional Info and things I don't know work
Car Wax
Prevent your bathroom mirror from steaming up after your next hot
shower. Apply a small amount of car paste wax to the mirror, let it
dry, and buff with a soft cloth. Next time you step out of the shower,
you’ll be able to see your face in the mirror immediately. Rub the wax
on bathroom fixtures to prevent water spots too.
Dish Soap
Using a light coating of dishwashing liquid can also do the trick.
Place a few droplets of soap onto your palm and add water. Wipe this
soapy hand across the face of the mirror and then wipe clean with a
paper towel. Voilà.
Mirror should remain clear for about a day; then you can simply repeat
the process. Using this method is also handy for creeping out other
shower users. Simply lather your hands in soap, write your message and
wipe.
From LifeHacker.com KeithS Comments:
Actually, dish detergent is better than bath soap because it doesn't
leave a soap scum film.
Walker Answers:
A good ventilation fan to remove the steam is the first step. Then you
need a heated mirror, or you can try to put a floor heating mat behind
the mirror. If the mirror is warmer than the tiles, most condensation
will form on the tiles rather then mirror.
How to Keep Mirrors from Fogging up With Steam