As the best basil is seasonal, you may wish to freeze batches of pesto for a welcome return to summer in the colder months. Before preparing the pesto, have a sterilized glass jar with a lid and a roll of plastic wrap handy. The jar should hold only slightly more than the amount of pesto you are making. Prepare the pesto. Leaving a little space in the jar, pack in the pesto and cover the top with a 1/4 inch layer of olive oil. Press a piece of plastic wrap evenly over the surface of the oil allowing the wrap to hang over the sides of the jar. Screw on the lid and freeze immediately. Pesto will keep for weeks, if not months, if prepared properly. Thaw to room temperature before adding to a recipe.