For those leaving at regular hours, I'd like to recommend a shuttle service that leaves from midtown Manhattan: Port Authority, Penn Station and Grand Central and runs every 20 minutes or so to La Guardia and Kennedy Airports:I take it repeatedly coming in and out New York; a single ride is $15 and it will take you all the way to your terminal and saves A LOT of stress.
--New York: 917 715 5664Mexico City: 55 4383 6914