I've been backing a FW for about 10 years now and find that the best advice is not to get in a hurry.  Take your time and don't be afraid to pull ahead and restart your approach. When I started, I placed a "Frisbee" at the location where I wanted the tires to be when parked and then backed the tires to that point.   Also, find that sweet point whee you start your "S" maneuver, place a Frisbee at that spot and measure from the site edge.  Now when you are approaching a new site, stop before you reach the site, measure the distance for the Frisbee and then pull the trailer to that point and you are ready to start backing.