There is no need to book car service months in advance. There is no discount for booking early. I use car services all over the country when in major cities for work. Most times I just call when I am sitting in the departure area of my departure city to make a reservation for arrival. They are always there waiting, never had a problem. I am not suggesting that everyone use this fly by the seat of your pants approach but a Call to a reputable company approximately a week prior to your departure date is all you really need to do. Give them your fight number and you are all set. They check to verify that your flight is on time so they aren't just sitting there waiting if your flight is delayed.