What William is saying is correct and is definitely key to backing into a site, especially if it is a tighter turn. I would like to add one little thing to it. After keeping to the left as you pass the entrance, and turning hard to the right as the front of the trailer passes it, THEN also cut hard to the left again just before you run out of room. What this does is kicks the back end of the trailer over to the left, aiming it straighter into the entrance, and leaving you with the trailer angled to the left of the rear of the vehicle.
I tried to draw a diagram for those that can't follow my babbling. Not a great drawing, but I hope it helps give the right idea. The first 5 steps are all forward to the stop, the last one actually reversing.