FYI - the skytrain at JFK runs in opposite directions for the "local" terminal loop and the trains going to NYC Subway stops (howard beach/Jamaica). "Subway" trains increase in terminal number (1,2 3 4 etc) and the local trains decrease (8,7...)
Simply take a howard beach/jamaica train and get off on the next stop. These trains run frequently especially during the day and it will take less than 5 minutes for a train to arrive and 1-2 minutes for the train ride. There is no cost or any difference whatsoever as long as you don't stay on the train past terminal 8.
there was one time in my experience when the local/subway trains were running in the same direction (both going 1,2, 3...etc) - last may due to construction of the jetblue terminal. Barring this possibility as a special occasion, this will save 10-15 minutes.