The courtesy wave is easy to do. It can range from a wide arc wave down to the single finger above the steering wheel, but how you do it doesn't matter. The important thing is that you are thanking the person who let you in for making your day easier and increasing the likelihood that you will be offered the same courtesy in the future.
Come on people let's not let the courtesy wave become a relic of the past! Use it whenever necessary and do your part in making the world a better place.