Do not use plastic to cover your plants. You will still get areas of freeze damage where the plastic touches the plants, plus it does not provide high quality insulation.
Do use fabric to cover plants. You will get good protection from frost and wind damage. I use old sheets and can easily find mismatched pieces for a dollar at thrift stores.
Make sure that the fabric cover touches the ground all around the plant. Any gaps will allow freezing to take place under the fabric. You can anchor the fabric using stakes made from old metal clothes hangers. Just cut off a section, then bend it into a U shape.
|Cover Touching Ground|
Remove the covers after daytime temperatures go above freezing. Why protect plants from freezing just to have them die from heat exhaustion?
Don't get lazy and think that skipping one night won't make a difference. It only takes an hour or two of freezing weather to kill a plant.