Keep your freezer set to -18°C. If it's mostly empty, keep some plastic containers full of frozen water inside to help it stay cold.
If your freezer has manual or semi-automatic defrost, defrost it on the regular.
A buildup of ice on the coils inside the unit means that the compressor has to run longer to keep cold, which wastes energy. Removing ice build up the unit's coils saves an average of 10% of a refrigerator's energy, which amounts to up to 60 kWh/yr.