Frosting is usually caused by the unit being short of refrigerant. On some of the a/c models imported from overseas (like LG models) the denseness of the evaporator can be a problem. The normal seating of the evaporator creates enough of an obstruction to the air flow through the evaporatior that icing can occurr. Running the unit on a higher fan speed can help to prevent it as well as running it at a warmer setting to give the a/c chance to dehumidify the air to reasonable levels before running it for maximum cooling effect. Running the unit with the evaporator coil blocked will usually lead to premature fan motor failure.