Freezing basil is one way of preserving it. There are several ways to freeze basil.
Which method is best depends on what you want to do with it once you take it out of the freezer.
In order to preserve its color, you can blanch basil prior to freezing it.
The easiest way to do this is to steam it over boiling water for one minute. Plunge it immediately into a bowl of ice water and then lay it on a clean kitchen towel to dry.
Now spread it out on a cookie sheet and put it in the freezer. Once it's frozen, you can place it in a zipper bag. It will thaw quickly when removed from the freezer. It will also taste good.
But not as good as fresh basil.
Another way to freeze basil is to chop it, place it into ice cube trays and cover it with water. If you don't want the extra water in your sauce, you can let the cubes thaw in a colander. It's easiest to just add them to the sauce still frozen.
A similar way of preserving basil would be to process the leaves with olive oil in a food processor. Spoon the resulting paste into ice cube trays and freeze.
Lastly, you could make a basil infused oil. Do this by warming half a cup of olive oil in a small saucepan. Add half a cup of torn basil leaves to the warm oil. Turn off the heat and let it cool. When it's cool enough to handle, pour the oil--minus the leaves--into a sterile jar. Store the jar in the freezer.
*Be sure to store herb infused oils in the freezer if you plan to keep them for more than a couple of days. The risk of botulism increases with the length of storage above freezing temperatures.
So which method of freezing basil is best? I like the ice cubes best because I mainly use basil to make pasta sauce. If your end use is different, you may prefer one of the other methods.