It's so annoying when you clean your glasses and they stay foggy.
I found a reliable way to avoid this using nothing besides a standard glasses cleaning cloth.
It's all about skin oil.
There is almost always natural oil on your skin, and you want to avoid transfering it to your glasses.
The most likely way how it would be transfered is via the surface of your cloth that you're trying to clean the glasses with.
How to wash your cloth to eliminate oils:
First, wash your hands super well with soap and warm water, to the point where your skin starts feeling different.
When you feel additional friction, that's how you know your skin oil has been temporarily removed.
Then grab your cleaning cloth and go crazy washing it under warm water (no cleaning agents should be added).
Rub it from all possible angles. Knead it against itself between your knuckles while squeezing with all your strength.
By doing this for a minute, you will have eliminated all unwanted pollutants.
Then just leave it to dry hanging in the air.
Congratulations, you have a clean cloth now!
How to keep your cloth clean, so it can be used more than once:
You can't keep it clean forever, but you can do some things to extend its usability up to a month or more between washes.
Most importantly, dedicate one side of the cloth to be lens-facing, and never touch that side with fingers.
I hope your cloth has different color/texture on both sides, so it is easy to distinguish them.
Otherwise, you can distinguish the sides by some persistent imperfections on the surface, or just memorize it while using it.
When you're not holding the cloth, it should be folded and stored in such way where the lens-facing side is hidden from touch.