WATCH the trailer wheels NOT the rear end. Put the wheels where you want them. Have a clear idea of where the wheels will go as you back in. Pre plan and visualize the path your wheels will take and mark where you want them to stop at. If the wheels deviate then you can correct before it is way off course.
Your assistant (if needed) must stay where you can see them and they can see obstacles on your blind side. Off to the side near the front bumper is best. If you cannot see them DO NOT MOVE the vehicle. This concept seems the hardest for my wife to grasp-she will disappear -I think to check the back for clearance- and then cannot understand why I stop moving.