Do not merely drape a sheet over the plant. Foliage touching the cover will be damaged by cold. Cloth gives more insulation than plastic. Remove the covers in the morning. Keep the soil around all plants moist during cold weather, whether in the garden or in containers.
Dry plants are easy victims of frost.
Keep citrus and avocado trees well supplied with food. The lush foliage may get frostbite, but it will protect the main part of the plant. If frost damages a plant, don't rush out with pruning shears to cut away the shriveled part. Wait until the plant buds out in spring, and then prune off the dead part. One way to lessen damage of overnight frost is by misting.