Opticians' tip: Never use a tissue to clean eyeglasses!
Tissues absorb dirt, thereby cause micro-scratches as the dirt particles are merely rubbed back and forth. When done multiple times month after month, the lenses lose their prescription strength. Experts call this "glasses blindness". Even small scratches on the glass that are imperceptible to the human eye can irritate and ultimately hurt the eye.
Aggressive cleaning agents
Window glass cleaner and aggressive household cleaning agents damage not only the glass coating but also the surface of the frames.
Wet cleaning cloths & eyeglass cleaning cloths with alcohol
Alcohol, which is usually contained in wet cleaning cloths, can damage plastic lenses. Therefore, use sparingly or not at all.
Too much pressure
Excess pressure can cause the frame to warp and can also damage the coating on the lenses.
Washing machine and dishwasher
Never, ever do this! Dishwashing detergents and salts can scratch your glasses. In addition, the heat could bend your frames.
3. How to properly store your glasses
Always store your glasses in their (clean!) case. When possible, wrap them in a microfiber cloth that is clean and dirt/sand/crumb-free.
It is a good idea to store the microfiber cloth in the spectacle case so that no dust or dirt can settle on it. However, microfiber cloths are machine washable: use a mild detergent, no fabric softener and wash at a temperature of maximum 40 degrees Celsius.
When your case isn't at hand, always place the glasses on the temples-- never lens side down! Resting your glasses on the lenses is a sure-fire way to scratch them. You should also avoid storing your unprotected glasses in pockets, jackets, and handbags.