Various. Water/soap, alcohol pads, ultrasound cleaning at the optical shop, replacement, etc.
Alcohol. Wipe with alcohol on cotton.
Dish cleaner soap. Place your eyeglasses in a solution of liquid dish cleaner soap and leave them there for an hour or so. Then use a cotton tip applicator to clean of the soil. This gets rid of all of it except sometimes there is some material underneath the pads next to the metal hold for the pad which might not be reachable.