The coldest air naturally settles at the bottom of the appliance and, unless it is absorbed by food, causes frost to develop. If you are running the freezer nearly empty, you should move your food to the lower shelves. Turn the freezer compartment off if it is not in use, to prevent ice building up on the walls.
The freezer should be set to 0 Fahrenheit or minus 18 Celsius to create ideal food preservation conditions. Any colder, and you risk causing frost or having the appliance run for prolonged periods. If your model does not have a digital thermostat, consider purchasing a freezer thermometer from a hardware store to help you set the temperature accurately. Temperature changes take about 24 hours to come into effect.
Every time the door to the freezer compartment is opened, moisture-laden air enters the appliance. During periods of high humidity, this moisture freezes to the walls, often at the bottom of the freezer. Double-check that the door closes properly; containers on the door shelves could be holding the freezer door slightly ajar.
Press the "Off" button on the control panel if the "QuickFreeze" feature is running; the temperature drops as low as minus 3 Fahrenheit or minus 19 Celsius when this option is selected. Designed to rapidly cool the freezer when a fresh batch of food has been added, "QuickFreeze" is not necessary for normal day-to-day use.