Plastic – Plastic is typically lightweight and easy to use while covering your plant materials. However, if it is laid directly on top of plants, there is no insulation and this is where damage can occur. In addition, heat can build up under plastic if the temperature rises and the sun is out. It is important to remove the plastic at that point, or ventilate. You can use stakes to prevent both of these issues while using plastic to protect plants during cold snaps.
Fabric – Using things that you already have on hand like old blankets, sheets, or landscape fabric can also be used to protect sensitive plants. The advantage of this is that air is trapped in the fibers and provides better insulation so plants are less likely to be damaged. If rain is on the forecast, this can weigh down the fabric and potentially cause damage, you can prevent this by using stakes to hold the fabrics up or removing beforehand.
Any material used should go down to the ground and held by rocks, pots, etc. The heat from the earth is what helps protect your plants. You may also layer materials for extra protection during particularly cold days and nights.